I've had probs with my 4x4 not engaging. I checked the vac lines and replaced the vac solenoid,Didn't help.When I turn the **** to engage the light COMES ON,click in the dash,and I can here the shift motor makes noise. I took the shift motor off,marked the internals to see if it was turning when I turned **** and it is. It seems to me that the shaft going into the trans case appears to have some "slop" in it like it doesn't know where to stop in 4hi or 4lo. I turned the shaft by hand to 4hi(it's marked on the case by the input shaft) and it worked fine. I guess my question is would it be the motor if my lights are coming on on the dash or does it sound like prob in the tcase itself.I thought maybe if the motor was going out it just might not be turning it all the way to 4hi or 4lo
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As suggested in other threads regarding the IWEs, can you spin the front axles when the truck is running and 4x4 is engaged? Have you checked the connectors to make sure you have power to the motor when the switch is in 4x4 positions? Maybe apply power directly to the motor to see if the tcase gets engaged. If direct power to the motor does work, than it's probably the motor, but make sure you diagnose the iwe's first.

 

if the light on the dash lights up, then your t-case is in 4x4. there is a limit switch inside the t-case that only gets actuated if it's physically in 4x4

I think your IWE may be the issue. hopefully the truck is under warranty?