So the issue basically feels like somebody throws the trans in park and the truck jerks to a stop? Or maybe like you slam on the brakes?
If this was coming from the rear axle gears it would have to be a serious failure, like metal chunks floating around inside the fluid. There are no electronic devices, actuators, etc... inside the rear axle that would somehow activate and cause a severe lock up. The only time I have seen a rear diff do this is a chunk of metal floating around in the gear oil gets jammed in the gears, and going forward for a second allows the chunk to release. However it would probably happen at other times and cause other very noticeable issues. The limited slip possibly binding would not cause a sudden lock up, especially when back straight up. If you were turning in the reverse and the limited slip was bound up the tires would still roll/spin and you would feel it, but the truck could still move with the inside tire just spinning/slipping/chirping on the pavement. Same basic comments about the transfer case. There is nothing in a transfer case that you shift to that locks it up...like a transmission has a park, but the transfer case does not.
It's kinda' hard to tell without actually being at the truck, but sounds like maybe an autopark engaging? Does this have an autobrake engagement if it senses something in reverse?