Do you get any 4x4 dash lights to come on?
So, the only place where the jeep will move is 2hi (lever all the way forward). Everywhere else the jeep acts as if its in neutral? Let's forget about 4x4 right now. Even in 4lo the rear wheels will turn and you can tell you are in 4lo vs 4hi. Then if the front wheels aren't turning we can get to that later.
I would take off the boot around the transfer case shifter. You should see this "gate". Its just a piece of metal that makes you move the shifter to the right and down to get to 4lo. Here is a picture, notice the "notch" cut into it.
When the shifter is resting on the notch, that should be 4hi. If you move it to the right and down, that is 4lo. If everything checks out up top, go under the jeep.
The linkage is adjustable. Although, if you can go from all the way up, to where 4lo should be. You should hit 4hi somewhere, even if its not adjusted properly.
Report back with your findings. Pics would be helpful. Maybe we can see something.