Originally Posted by DaemonForce
A Wrangler with a 4.0L can't get out of 4WD...
I don't understand this comment. OP has 2H and really should be using it most of the time. Use of 4WD should be in low traction environments only. TJ TCs are not AWD. There is no center diff there and so your front and rear driveshafts are locked together when in 4WD. If you aren't in a low traction environment where the wheels can slip to compensate for the axles moving at different speeds things will bind up in the TC.
Next OP has mechanical linkage not vacuum. That is likely the problem. It actually won't look like either of the pictures though.
OP, first if you are driving on the dry road in 4H, stop. Very bad for your TC. Get under your jeep and use the lever attached to the TC to get it back in 2H. Next, follow the transfer case linkage to see if broken. Start either at the tub under handle or at the TC and follow the system of rods and levers. It is a pretty Mickey Mouse setup with a bunch of rods and bushings. Very common for the bushings to wear out and the linkage to disconnect. There is a rebuild kit that the dealer sells but most people take it as an opportunity to go to one of the aftermarket linkage options. There are hard links where you end up with something like the second picture that was posted (Teraflex for example makes some thing like that), but the best option is a push-pull cable option from Novak.
If it turns out not to be the linkage and and you can't manually shift it back into 2H then you've probably buggered up the inside the TC by driving it in 4H in a high traction situation. Answer then is you have to open up the TC, figure out what's broke there and it may or may not be fixable.