You can get the limited slip / posi traction additive lube from the auto parts stores or from the parts dept. at your local GM dealership. If this is what you are looking for. I bought the gear oil before that said it had the additive in it. But i ended up buying a bottle of the additive to add to it.
Depends on what the unit is....
Stock may need the bottle.
The detroit locker none.
The Eaton Trutrac, definitely NOT. In fact, they suggest oil w/o, or very minimal add in it.
Ive seen it in a couple mom and pop type napa stores, most parts stores should have some type of additive if not that brand. Theres an amsoil guy at my local track that always has it