Any rear end that allows the "other" tire to spin is a "limited slip".
Posi-traction is just one of the types of LSD's. A locker is another.
Posi-traction is good because it only engages when the other tires loses traction. So for normal driving, only one tire gets traction.
But a locker engages both tires all the time. There are some that dis-engage when you are in a turn, which is the only kind you should comtemplate if locker is what you want.
From my experience, a Posi LSD will engage the other tire quickly enough to not make much of a difference. But if you are for "all out drag", then a locker is the way to go.
Scott