Positraction is the Chevrolet name for a limited slip differential, just like how Chrysler used Sure-Grip for theirs. The first year for Positraction in a GM truck was '59, the used the Spicer 45 GMC axle in both Chevy and GMC trucks wehn equipped with that option. An Eaton based Positraction differential came out in '60 for Chevy, and was used until '62, although I have sen code listings for it in '63 as well. Seems those Posi 3rd members are soemwhat rare, I've only ever sen one.
I'm going to venture to say that if you do indeed have a Posi dif, it is most likelya later rearend out of a '70s truck, when they became more common.
