Ive got a 99 automatic 4 wheel drive!Truck is driving fine and I go to pull into the gas station maybe going 15. And truck just makes a huge clunk and just slams to a stop! I get pulled off the road I put it in neutral! And rear tires are locked up! What's wrong with it? Rear end? Transfercase? Tranny?

 

Got to pin point it, but if you put it in neutral and it moved fine id look into the tranny. Ive had the parking pin move on me before and did that

 

if the wheels are still locked up you can remove the drive shaft. then you can see if it is in tranny, transfer case, or rear end.because if it rolls with ds removed it wont be in differential