Why are you ditching the 4.11's? That's the second best street gear.
Anyway, you have to look on the drivers side of the third member. There will be a casting number there. The last three digits is what counts.
As for the sure grip (I don't want one of the coffee crew kicking my can over the "posi" nomenclature) the quick way to tell what you have is to look at the diff and count the ends of the cross pins. 2 cross pin ends and it's the cheap unit. 4 cross pin ends and it's the good unit.
