Typical car cruise street hoopie. Lol
There are different names for a Limited Slip Differential. The difference is in the name, not much else, though there are different ways of building a limited slip differential. Chrysler called it a Sure Grip, Dana used Power Lock, Chevrolet was called Positraction, Buick- Positive Traction, Packard- Twin Traction, etc. So a Posi is a limited slip where as a limited slip MIGHT be a Posi.
Limited-slip differential - Wikipedia Scroll to near end of the article for cool "posi" names.
Kind of like Vice Grips are a brand of locking pliers, but everybody calls them vice grips if there is a knob on the end of the handle. Or Linesman Pliers seem to all be called Kleins.
edit; now I know that a Trans-Am would not have had a "Posi" from the factory any more than my Dart would have. It would have a Safe-T-Track. Fodder for irritating car show twits.