Jim:
If you have a Gen2 and have had the 998 recall I can make a good guess. Look back for a post I made about 8 months ago, it discussed the the two bolts that hold the differential carrier to the frame. In a nutshell the oem bolt and bushing **** (loose fit) to begin with. When the recall is done they use an under-size bolt (for no reason). The result is that the carrier is now very loose and floats within the frame. All that keeps it from moving/floating it is the friction from the compression on the bolt, however, that is not enough to keep it from moving around when the car brakes or torques.
On my car, I ended up making a custom bushing for the carrier. My car was actually 1.5 inches higher on one side in the rear until I replaced the bushings. No one makes a bushing other than the oem, which you do not want. The reason that this goes unnoticed is that it cannot be seen and typically you don't look for the fit in this bolt/bushing combination. If you take the 998 recall bolt and slide it into a car with 20,000 miles, you will be shocked. Mine had almost 3/16" play. Magnify the 3/16" by a 36" fulcrum and that is the movement you get at the rear.
Then again if you have a Gen 1, I wasted your time.