Broken gears inside the diff or the pin turning the gears.
Or is the rear locked up and blocking movement? Take the center diff out and look, pretty simple to do, might as well start getting your hands dirty. This hobby is 70% fix/repair/change/upgrade.
This is a really dumb question but I'll ask it anyways, Do you have both of your rear driveshafts and are they connected? The last time my truck turned front wheel drive as after a really hard landing and that was the cause. If they're both there then blame the diff drive pins. EXB, might have busted a back off of one of the LSD gears.