Have you had any work done on it where they would have replaced the gear oil in the rear end? If so then they may have not put in the posi additive. If not then I would remove the rear cover inspect for any metal and other debri and put in new fluid. If you do this yourself then make sure that you have fluid with posi additive already in it otherwise you need to get a bottle of posi additive. I use Lucas 75-140 and then mix in 25% of the oil stabilizer from lucas too. Lucas has the posi additives already in it and by putting some stabilizer in it you make it cling better to the clutches and gears.