Sounds like the diff may not have been installed properly. Get two straight 8' 2x4's and a friend, and line them up so they are parallel to the body. Place the board against the rear tire so it touches both sidewalls above where the tire bulges at the bottom, and so it almost touches the rocker panel. If you have a couple of coffee cans, you can leave it in position while you do the same to the other side. Now compare the two sides to see if the boards are truly parallel, or if they both point to the right or left.