I am new to this..I just bought a fx4 I couldn't test out the 4x4 prior.. It will not go into 4wd, the light comes on, the shift motor puts it in, but the wheels will not spin. I jacked it up, while the truck is shut off the fronts lock in as they should, start it, and the vacuum kicks and and unlocks them, so if i spin the tire the axle does not spin. I switch it to 4x4, and the axles still do not spin, as in the vacuum does not release.

Also, with the truck off, key on, i switch to 4x4, the motor locks it in and the wheels spin one forward and reverse like they should.

I am thinking a bad solenoid?

 

https://www.ford-trucks.com/forums/8...then-some.html

welcome to FTE. this is likely the remedy you need.

also if you notice in the center of the page is a "search" button. this usually helps speed up q&a