I'd guess that the problem is with the U-joints or the rear end.
I'd park the car somewhere level,put it in neutral,set the emergency brake and turn the driveshaft back and forth by hand.Is there play in the rear end? does it feel like it stops right away against the gears or does it feel like it still moves a bit after the first contact? and can you move the end of the driveshaft up and down where it bolts to the rear end?
Those are signs the rearend has problems and if it's the stock 7 1/4 rear,it's very likely.
