Depending on when your factory GM posi came from, its either an auburn style or a clutch style. If its the later Auburn cone style, skip it. They are not rebuildable and most have enough miles on them that their lives are limited at best.
If you get the Auburn cone style, forget the $100 exchange thing. You are responsible for shipping both ways (including insurance), and depending on where you are, that could be $50 plus another $10. I just don't like the Auburn unit. If you're worried about price, trust me... the Eaton could cost twice as much and I'd still buy it.
Eaton style limited slips are a totally rebuildable, tuneable system that (in my opinion) are way beyond anything that Auburn ever built.