This video shows you how to tell if your rear wheel drive car or truck has a limited slip differential, and also what gears it has. It is quick and all you do is read the tab on the differential pumpkin, or lift the rear and spin the tires... While moving one tire by hand, If the tires move in the same direction (or one doesnt move), it means that it is most likely a limited slip (posi traction) rear end. A limited slip rear uses a special additive to the gear oil when you do a gear-oil change, and it is necessary or damage could occur. An open differential (like the one in the video) does not use the special additive and the tires spin in opposite directions when one tire is spun.