Positraction and Locking differentials are different beasts. Posi (and limited slip - generic name) use clutches to reduce slippage. Locking differentials mechanically lock both sides together. Searching Google for locker and positraction should come up with some good explanations of the differences.
These axles have survived a long time, often being abused. If you follow the manual directions, you should be fine for a long time to come.
