Liftgate wont open, have to use the key, but if I press the key to close the rear lift gate it will close it.

Door ajar switch possibly . The lift gate not working is a different matter . Could be any number of thing's .
Power Liftgate System Components
• Liftgate motor assembly
• Liftgate latch
• Liftgate module (LGM)
• Liftgate side upper sensor - left
• Liftgate side upper sensor - right
• Liftgate handle switch
• Liftgate close switch
• GMLAN serial data link
• PWR LIFT GATE 30 amp fuse
• PLG 10 amp mini-fuse

Your best bet take it to a qualified repair shop that has the diagnostic tool that can communicate with the lift gate control module , too check for DTC'S - diagnostic trouble code's .
Liftgate Module
The liftgate module controls the liftgate motor, liftgate clutch, and liftgate latch based on various inputs. The liftgate module also performs diagnostics on these inputs and outputs and will disable liftgate functionality and set DTCs should a fault be detected. The liftgate module is secured to the motor assembly at the left rear of the vehicle. The module requires a liftgate position learn procedure be performed when replacing. This allows the module to learn the full open and full closed positions of the liftgate.

DTC B153A 00: Liftgate Latch Switch Signal Circuit
DTC B153A 08: Liftgate Latch Switch Signal Circuit Signal Invalid
DTC B153B 02: Liftgate Open Switch Signal Circuit Short to Ground
DTC B153B 05: Liftgate Open Switch Signal Circuit Open
DTC B153E 02: Liftgate Position Sensor Signal Circuit Short to Ground
DTC B153E 05: Liftgate Position Sensor Signal Circuit Short to Battery or Open
DTC B153E 08: Liftgate Position Sensor Signal Circuit Signal Invalid

This is not a job for a diy'er or vehicle owner .

Try the Suburban forum, where other people had the same problem at : http://www.tahoeyukonforum.com/threads/rear-hatch-wont-unlock-lock.54757/
I hope this helps you out!

How do you know it need's a switch ?
Rear Hatch/Gate Description and Operation
The lift gate can be opened by operating the switch on the instrument panel. Battery voltage is supplied to the lift gate actuator through the body control module (BCM). The BCM will pulse the lift gate actuator when a release input is received through the key fob, or the instrument panel switch. Did you test voltages ? Check for BCM DTC'S - diagnostic trouble codes ?
Circuit/System Description
The lift gate switch is an input to the body control module (BCM). The BCM supplies battery voltage to the signal circuit and when the lift gate switch is activated the signal circuit voltage goes low. The BCM controls the lift gate release actuator assembly through the LIFTGATE relay. When the lift gate switch signal is activated, the BCM enables the LIFTGATE relay by applying battery voltage to the lift gate relay circuit and the relay is energized when the BCM grounds the LIFTGATE relay control circuit. When the relay is energized battery voltage is supplied to the lift gate release actuator assembly through the lift gate release actuator control circuit.

Open the liftgate. Remove the liftgate applique. Refer to Liftgate Applique Replacement in Body Rear End. Remove the back up lamp socket (1) from the liftgate applique by turning the socket (1) counterclockwise. Remove the back up lamp bulb from the socket.
Lift gate Applique Replacement
Remove the lift gate trim panel. Refer to Lift gate Trim Panel Replacement .
1
Lift gate Applique Nuts (Qty: 5)
Tip
Disconnect the electrical connector.
Tighten
10 N?·m (89 lb in)
2
Lift gate Applique

Unless you have electrical diagnostic skills , I suggest you take this to the dealer ! The lift gate can be opened by operating the switch on the instrument panel. Battery voltage is supplied to the liftgate actuator through the body control module (BCM). The BCM will pulse the lift gate actuator when a release input is received through the key fob, or the instrument panel switch.
The body control module (BCM) receives an input from the liftgate ajar assembly. The ajar assembly is normally open when the liftgate is fully closed. When the liftgate is opened, the ajar assembly contacts close providing a path to ground. The driver information center (DIC) illuminates the rear access open message when the gate is open. The instrument panel cluster (IPC) receives a serial data message from the BCM indicating the liftgate ajar status. If this message is displayed and the vehicle speed is greater than 8 km/h (5 mph), a chime will sound.
It could be ajar switch is bad .

I have a 2008 Suburban with the power liftgate disabled. I have had the power latch stop working, and the BCM report that the rear access open when it is closed (both gate and glass). Somehow the liftgate module thinks the gate is open, so I cannot open the gate at all, and the latch and rear wiper will not work. To reset the control module I pulled the liftgate fuse (#20) in the instrument panel fuse box, turned the car on, off, reinstalled fuse, turned car on, unlocked doors using drivers side switch, and the power latch will function again - at least for a while.

It can be faulty switch or its issue with lift gate latch.Its got stuck up hard or cracked inside.It has to be inspected.-----------
There are few help links with proper solutions for these problem.I suggest you to refer the links.
Click the link below:----
2007 Mercury Mountaineer: Liftgate will not Unlock or Open? http://whoisbyme.blogspot.in/2012/06/2007-mercury-mountaineer-liftgate-will.html
------------
Adjust Hydraulic Lifters? http://schematicsdiagram.blogspot.in/2012/04/adjust-hydraulic-lifters.html
-----------
How to open rear lift gate? http://repairhelpcenter.blogspot.com/2011/12/how-to-open-rear-lift-gate.html

--------- Power Liftgate Shuts Down http://whoisbyme.blogspot.in/2012/07/power-liftgate-shuts-down.html
------------ Power Lift Gate Will Not Remain Open http://whoisbyme.blogspot.in/2012/07/power-lift-gate-will-not-remain-open.html
----------- Power Lift-gate will not Operate http://whoisbyme.blogspot.in/2012/07/power-lift-gate-will-not-operate.html
------------- Liftgate will Not Stay Up Open? http://whoisbyme.blogspot.in/2012/07/liftgate-will-not-stay-up-open.html

---------------- Rear Door will not open http://whoisbyme.blogspot.in/2012/07/rear-door-will-not-open.html
------------ 2000 Toyota Corolla: Rear Door wont open from outside or inside? http://whatisbyme.blogspot.in/2012/05/2000-toyota-corolla-rear-door-wont-open.html

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These details will help.
Thanks.

POWER LIFT-GATE OPERATION - To open the optional power liftgate, push the button on the instrument panel , press on your five-button key fob (if equipped), or press and release the control located in the left rear quarter panel (Figure B). To unlatch the liftgate glass, push the button ) under the center of the license plate lamp shield, or press on your five-button key fob. Do not open the liftgate or the liftgate glass in a garage or other enclosed area with a low ceiling. Keep clear of the liftgate when operating the rear switch. To open the liftgate manually, pull the handle located on the liftgate to release the liftgate from the vehicle. The power system will take over when you release the handle. To close, make certain the rear window is closed, and then pull down the liftgate a few inches to activate power closing. Closing the gate rapidly or more than a few inches may activate the obstacle detection feature and stop the power system.------------ Also click this link for more help:--- http://technoanswers.blogspot.com/2011/05/power-liftgate-on-will-not-open-on.html ------------ This should help.Keep updated.Thanks.Helpmech.

how to i open a stuck lift garte on a ford winstar 2000 lock broken too