front wheels wont engage in 4hi or 4lo

The way the system works on your truck is, IF the actuator is OK and getting the proper engagement command, wheel rotation must be present for the actuator to engage the drive axles, in other words if you are just lifting vehicle off the ground and engaging 4x4 the actuator cannot do its job in locking the axles in until drive axle rotation is present for the pin to engage. I assume (and I hate that word) that all circuitry for the actuator and transfer case and the mechanical condition of transfer case and front differential has all been verified as sound and functional. Best of luck Tim

If the light on the dash indicates the 4x4 shift has been completed (on steady).. while driving in a parking lot, turn the steering wheel hard right or left, you WILL know then if it is working!! The 4x4 system can be scan tested if a problem exist! Which will be indicated by a flashing light on the 4x4 switch followed by no light at all. If it will not engage the most likely suspect is the front axle actuator even though it seems to be working!!! Have it scanned.