I would suspect the vacuum disconnect motor or the vacuum solenoids for the disconnect motor ( or the vacuum lines ), but best to confirm
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.
With the engine running ( and wheel chocked, parking brake on ) have someone shift into 4WD, and watch the shift motor to see if it operates
It is on the front axle, looks like this
If the disconnect motor does not shift, you can check the vacuum on the lines with a vacuum gauge. Blue line is 4WD operation, pink line is 2WD operation.
The vacuum solenoids are on the firewall, and look like this
This is the block diagram for the ESOF vacuum operation of the disconnect motor.
This is assuming you have checked the fuses for the transfercase shift motor, and the vacuum solenoids.
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