stuck transfercase

I assume you've checked out the drive-shafts and the transmission/transfer-case and so forth under the truck...
Made sure everything is sitting normal, not falling off/etc under there.
If that's all good, how about the shifting linkage, can you make it shift from the linkage under the truck? If not, then it could be an issue inside... I'm no expert as to what it could be, but someone else here should have an idea.

Sounds like the range fork is in need of replacement. Nylon in the pump strainer (near the drain plug) will confirm that.

Is the transfer case shifter rusty/stiff? If so, it preloads the forks, causing accelerated wear on the nylon pads, then the forks fail.

Mine has stuck in low range sometimes, just had it apart and was missing two brass blockers on the snycro, nylon looked perfect

Is the linkage rusted? They seem to work fine back and forth but seem to seize up moving left and right going from hi to neutral and low.