Why won't the rear sliding door open completely?

I'd recommend starting by turning the power door function off and trying to operate the door manually. If it is difficult to slide, lift up on the door while trying to move it; if it then becomes easier to move, chances are that the center roller has failed. The center roller bears all of the weight of a sliding door as it moves through the center track located below the rear side window glass. There are also an upper and lower roller, but they don't bear as much weight as the center roller and act more like guides. The lower roller area does tend to catch debris, usually from kids (crayons, cereal, etc), so it would be good to clean that out before settling on the center roller as the issue. You can read more here: https://www.yourmechanic.com/question/sliding-door-won-t-open

It’s probably the door latch on that door

Could be individual switches or the rear may have child safety locks on them--the clue will be usually you cannot open rear doors from inside the vehicle---it (the child locks) will in many cases stop rear windows from working----consult the owner manual or look at inside of rear doors for the slide switch at each door under and near the rear of the armrest.

Did you ever get the sliding door handle fixed? I have the exact same problem and would love to know how you fixed it. Please email me at [email protected] thanks!

The proper way would be to look at a wiring diagram an test the electrical circuit's with a DVOM - digital volt ohm meter . Do you know how to test automotive electrical circuit's ?
Electric Testing Techniques You Need to Know
The Trainer 32 How To Read An Automotive Block Wiring Diagram
Free wiring diagrams here http://www.bbbind.com/free_tsb.html
Enter the vehicle info. year , make model and engine size . Under system click on body & accessories ,then under subsystem click on power sliding door . Click the search button then the blue link. Looking at the diagram you will see a control module is used .
Also reading factory service info. on how it works would be a good idea.

Rear Side Door Actuator Control Module
The rear side door actuator control module is capable of on-board diagnostic functions described in On-Board Diagnostic Features . The rear side door actuator control module monitors the door position at all times including during manual operation. After activating the on-board diagnostic modes, loosing battery voltage, or ground to the rear side door actuator control module the Power Sliding Door (PSD) Re-Initialization procedure must be performed in order for the rear side door actuator control module to correctly determine the door position.
Power Sliding Door System Description
The power sliding door system(s) can operate correctly only if the power sliding door is in proper mechanical condition. Any mechanical problems will affect the power sliding door operation and must be repaired before attempting to diagnose the power sliding door system. Manually open and close the power sliding door to determine if the door moves smoothly and without excessive resistance or binding. The power sliding door must also latch and unlatch without the use of excessive force. At any time during power sliding door open or close operation, excessive resistance to the door movement will cause the power sliding door to stop and reverse the direction of travel.
The power sliding door will operate only when the power sliding door on/off switch in the overhead console is in the on position. At anytime during power sliding door operation the on/off switch may be turned to the off position and the system will quit any function in process. The power sliding door will not open unless the transmission shift lever is in the park position and the door is unlocked. When the sliding door security lock is in the locked position the power sliding door will not open from the inside. The power sliding door close function will operate when the vehicle is in gear. The power sliding door module uses the VSS signal to determine if the vehicle is moving during a power sliding door close function. The momentum of the vehicle moving forward adds resistance to the door closing operation. When the rear side door actuator control module determines the vehicle is moving forward while closing the door, current flow through the actuator motor is increased instead of being reversed. The VSS signal circuit is supplied battery positive voltage through a resistor then monitored within the rear side door actuator control module. The VSS signal circuit is then pulsed to ground, or pulled low by the powertrain control module in relation to vehicle speed. The sliding door alarm will sound repeated short tones whenever the power sliding door system is turned on and the door is open while the transmission is out of park.
When the power sliding door system is on and manual operation of the power sliding door is detected the rear side door actuator control module activates the power assist feature. When the power assist is activated the power sliding door will continue to open or close without any further manual effort.
The power sliding doors may be operated using the Remote Keyless Entry System. The keyless entry transmitter will have one OPEN/CLOSE button with a van symbol on it for each power sliding door. The power sliding door ON/OFF switches in the overhead console must be in the ON position in order to operate the power sliding doors using the keyless entry transmitter. When a right hand power sliding door OPEN/CLOSE function is activated using a keyless entry transmitter the exterior lights will flash once indicating the RKE module has received the OPEN/CLOSE command. When activating a left hand power sliding door OPEN/CLOSE function with the keyless entry transmitter the exterior lights will not flash.

On-Board Diagnostic Features
The left and right power sliding door systems operate as independent systems. The on-board diagnostic modes are entered and operate the same in both systems.
The power sliding door systems are capable of the following on-board diagnostic functions:
• Diagnostic Mode 1: An audible signal indicates the diagnostic trouble codes stored by the rear side door actuator control module.
• Diagnostic Mode 2: An audible signal indicates when inputs to the rear side door actuator control module are active.
• Diagnostic Mode 3: Performs an output test of the sliding door unlatch actuator.
Entering On-Board Diagnostics
The on-board diagnostic modes can only be entered in a sequential order from modes 1-3.
• Enter the right power sliding door system on-board diagnostics using the right B-pillar open/close switch and the right on/off switch in the overhead console.
• Enter the left power sliding door system on-board diagnostics using the left B-pillar open/close switch and the left on/off switch in the overhead console.
1. Both power sliding doors must be closed and in the latched position.
2. Place the ignition switch in the OFF position.
3. Place the on/off switch in the overhead console to the OFF position.
4. Press and hold the B-pillar power sliding door open/close switch.
5. While holding the open/close switch, operate the on/off switch through 10 ON and OFF cycles within 10 seconds stopping in the on position.
6. The sliding door alarm will sound 1 long tone indicating diagnostic mode 1 is active.
Diagnostic Mode 1
Upon activating diagnostic mode 1, DTCs will begin to be signaled by the sliding door alarm. The DTCs are 2 digit codes and the codes are represented by the number of buzzes for each digit of the code number. For example a DTC 12 is represented with the following sequence: buzz, pause, buzz, buzz, long pause. Each DTC will be signaled 3 times before proceeding to the next DTC. Refer to Diagnostic Trouble Code (DTC) List . The DTCs will continue to be repeated in this manner until the on-board diagnostics are exited or mode 2 is entered. When exiting the on-board diagnostics all stored DTCs will be erased from the rear side door actuator control module memory.

This may or may not work depending on exactly what is wrong with the door, I cant tell what is wrong without looking at it directly however:
You can try starting with all doors closed and unlocked, keys in hand disconnecting the battery and letting the vehicle sit for 2 min or so, then reconnect the battery. Put the key into the ignition and turn it to the on position but do not start the van. Next press the button to disable the power sliding door for the drivers side, then manually open the door fully, wait 5 seconds, then close it manually fully. Press button the button again to re-enable the power sliding door, then try to use it with the automatic feature either by pulling the handle or using your remote, this may or may not work depending on if the door has somehow become de-initialized (it should work now) however if something is broken inside the door closing mechanism or door closed/open sensors, the door will continue to malfunction.

POWER SLIDING REAR DOOR WON'T OPEN ON A 2004 FORD FREESTAR WHEN THE BUTTON IS PUSH EITHER ON REMOTE OR CONSOLE. ONE DOOR DOES WORK.

From the back of the front door slide your hand in the sliding door crack and find the child-safety-lever. Push the lever up and/or down while moving the sliding door manual lock/unlock button to the up position. Also works on the non-powered sliding door.

I would start by pulling the door panel off the inside. This will give you a look at the lock and handle mechanism to see what is amiss. Suspect a broken, fallen link that may now be lying in the bottom door channel and retrievable with magnetic pick-up tool. Get a good trim removal tool since a panel that large is easy to mar if you try pulling it with a screwdriver. Have a helper assist with the job and start by pulling the rear and middle seats if you can. To cheat on this, you may want to pull the panel on the opposite door to see how everything is supposed to look.