75W140 synthetic diff oil. If you have a limited slip, before you purchase the "friction modifier" check the label of the oil you buy. Some of them come with the modifier as part of the formulation. If it does not state that the fricition modifier is already in it, it probably isn't.
Start with adding 4-6 ounces of modifier, drive the truck for a couple hundred miles. On a long trip where you know the rear diff oil is up to temp, find a parking lot. Come to a full stop and cut the wheels to one direction (doesn't matter which). Slowly roll into the throttle. If you feel a "chatter" add one more ounce. Drive it like this for a few hundred miles and repeat the test. Never add more than 8 ounces of the modifier. If you still get chatter, your limited slip clutch pack needs to be rebuilt.