re: Truck Rear Differential question

Check to make sure the differential is, in fact, limited slip and then go to your local auto parts store and get 3 - 4 bottles of oil (my tacoma uses 75w90, and I would be willing to bet yours does, too) along with a tube of friction modifier and change it yourself. If it continues, then research your next step. The plug should have a magnet on it - if there isn't metal on it I would probably look in another direction.