2003 f150 4x4 don't engage. everything down to the front differential seems to be working

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Check for power to the front differential actuator. This actuator slides a gear in position to lock the differential to drive the front wheels. If you have power and ground to the actuator when in the 4HI position, the actuator has gone bad.The 4LO is only engaging at the transfer case, but the front wheels are still not engaged if it is not working in 4HI.

The Selectable Four Wheel Drive (S4WD) Front Axle consist of the following components:
• Differential Carrier Housing
• Differential Case Assembly
• Inner Axle Shaft
• Intermediate Shaft Bearing Assembly (located on the right side of the oil pan)
• Electric Motor Actuator
The front axle on Selectable Four Wheel Drive (S4WD) model vehicles uses a disconnect feature mounted on the right side of the oil pan in order to engage and disengage the front axle. When the driver engages the 4WD system, the Transfer Case Control Module sends a signal to the electric motor actuator to energize and extend the plunger inside. The extended plunger moves the clutch fork and clutch fork sleeve across from the clutch fork outer gear that is splined to the right side wheel drive shaft to the clutch fork inner gear that is splined to the inner axle shaft. The locking of the two gears allows the axle to operate in the same manner as a semi-floating rear axle. A propeller shaft connects the transfer case to the front axle. The differential carrier assembly uses a conventional ring and pinion gear set to transmit the driving force of the engine to the wheels. The open differential allows the wheels to turn at different rates of speed while the axle continues to transmit the driving force. This prevents tire scuffing when going around corners and premature wear on internal axle parts. The ring and pinion set and the differential are contained within the carrier. The axle identification number is located on top of the differential carrier assembly or on a label on the bottom of the right half of differential carrier assembly. The wheel drive shafts are completely flexible assemblies consisting of inner and outer constant velocity CV joints protected by thermoplastic boots and connected by a wheel drive shaft.

How To Install Repair Replace 4x4 Front Axle Actuator Chevy Trailblazer...

You probably have a front drive splined disconnect assembly that has gone bad. This assembly engages the front differential when 4WD is selected. Below is a figure of the item, it is on the passenger side of the differential area.

Its a long procedure.

Go through the related help links,similar to your issue:---
Click the link below:---
How to remove Front differential? http://howtobyme.blogspot.com/2012/01/how-to-remove-front-differential.html

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Timing Belt, Cover, Sprockets, & Seal replacing ? http://technoanswers.blogspot.in/2012/02/timing-belt-cover-sprockets-seal.html

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Crankshaft Front Seal? http://schematicsdiagram.blogspot.com/2011/12/crankshaft-front-seal.html

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Rear Main Seal Leaking? http://whatisbyme.blogspot.in/2012/05/2003-lexus-rx300-rear-main-seal-leaking.html

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Differential Chatters? http://whatisbyme.blogspot.in/2012/04/1996-toyota-prado-differential-chatters.html

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4x4 front differential will not engage the axle? http://technoanswers.blogspot.in/2012/02/4x4-front-differential-will-not-engage.html

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CV AXLE stuck in Transmission? http://technoanswers.blogspot.in/2012/04/cv-axle-stuck-in-transmission.html

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These will help.For more help,keep updated.Thanks.

check for codes with scanner it most likely is the abs sensor on the rear differential housing that is bad easy fix

There is a gas canister on the front differential that engages the hubs for 4wheel drive. make sure it is functioning properly. It works by way of heating the gas in the canister which forcess a rod to slide and engages the front diferential. During the cold months it takes longer to heat the gas in the cylinder and sometimes won't allow the diff to engage. Check to make sure it is functioning it is located on the inside right hand side of the front differential.

Lift and support the front of the truck so the front wheels are off the ground ( take safety precautions, chock wheels, jack stands, etc )
Key on, engage 4x4 and try to turn the front drive shaft.
If it turns it is a problem with the transfer case.
If it doesn't it is a front axle engagement problem.

I don't want to assume that the front axle works correctly, but it sounds like you got past that bit of testing.

I had the same problem with mine. Everything seemed to be working meaning the actuators and switches but front wheels would not turn. I took it to a guy's shop I know and he drained the front differential' looked like there was water in the fluid maybe from condensation or something. It has been so cold where I live he thinks it was freezing the pin or fork inside. New fluid and it is working fine now.

Tranny slippage can be caused by several things (each contributes to a varying degree, if at all)

1. cold parts are smaller > > > don't seal properly
2. cold fluid is more viscous > > > don't flow properly
3. bad fluid can cause build-up > > > slow actuators & clog fluid passages
4. worn parts > > > don't seal properly

Result: low fluid pressure pressing on bands/clutches = slippage

The good news: as everything warms up, tolerances normalize, system leaks seal & fluid flows more readily creating enough pressure to keep bands/clutches from slipping excessively (normal operation)

As for the 4WD not engaging, I would be willing to bet the front differential actuator is frozen/stuck (assuming it has worked recently)