Mike gave you the major part of the picture. I would only add that, typically, with the 28 spline, the stock axle shaft is the weak link. If you take a rear end apart that has had some hard drivin' done to it you'll often see the splines have a bit of a "spiral" to them. This is a precursor to the axle shaft snapping and means the axle shaft should be replaced. I've seen this more often on 9's than 8's, which may not mean anything other than 9's get abused more.

One other note to go with Mike's comment on braggin' rights. I've often heard, or read, where people claim to have a 9" when they actually have an 8". Either they don't know any better, or they're doing the same thing they do about their hp rating.