Rebuild the shift motor.
It's easy.
There is a rubber grommet in there that gets old and cracks and falls apart. Sometimes the piece of rubber can cause the motor to stick, sometimes it is the water and dirt that gets in there once the grommet is toasted that cause the motor to stick. IN either case it is simple to rebuild. I have redone 3 of them to date and all 3 work perfectly still. Just make sure to mark the motor case and the top before you take it apart so you can lign it back up exaclty.
Also sometimes they actually turn a little in their mount and this can cause it not to work, a tap with a hammer can get it to shift, but it's on it's way out.
Basically if you push the 4x4 button and NOTHING happens the motor is bad 99% of the time. If you push te button and it flashes twice but nothing shifts, 99% of the time the motor is toast. If you have high range and then push the button to low range and the truck is in neautral and nothing happens, guess what, 99% OF THE TIME THE MOTOR IS TOAST.
The autohubs can also cause 4x4 shift problems, but those things should already be in the trash if you are going off road.........
Ford and junk yards make a killing replacing these motors at $400+ ea, hell I could probably quit my job and spend all day replacing them for that much. Mine has got 230K miles on it and I havent had to rebuild it yet, but I will. Seems the older ones hold up better (mine is an 88, I have rebuilt a 91, 93, and a 94)
The full size trucks use the same motor, if you ever see one of these suckers still on the truck ina junk yard, grab it! it is rare to find a truck with one still there but if you do BUY IT!!!!!