I had the same problem. It seems to come from the front, but really it IS the clutch packs in the limited slip differential. I fixed my problem by changing the rear differential fluid and adding the required Ford friction modifier. I changed my fluid out completely since I had over 120,000 miles, but it is expensive stuff and may not be necessary in your case. Many people have tried adding just a bit more friction modifier (about 1/4 of the small container) and this solved their problem. The latter is a $5 fix and you have nothing to lose by doing it. Use a 3/8 ratchet and remove the fill plug from the differential. If necessary use a suction tube to remove a small amount of gear oil so you can add the friction modifier (you might not have to do this if the diff still has room for a few ounces. Hope this helps. | 
