12-Bolt Question

[C15_Sierra]

I bought my '80 Sierra with a 305/TH350 and 2.73 gears (yea, old man's truck). The motor and tranny have been replaced already with a 5.3/4L60E so it's time to focus on the other bits to spice up the truck. One of the things I'd like to do is get rid of the 2.73 gears (who wouldn't) and put a posi-trac out back. I'm thinking 3.73's since I have OD now.

So my question is...did the PO at some point swap out the rear axle? I don't know much about 12 bolts, but I've read that 2.73 came in passenger cars but not trucks. FYI I measured out the # of driveshaft rotations per wheel rotation (2 and roughly 3/4 = 2.73). Is there anyway to verify if I have a car or truck 12-bolt?

And my next question is, assuming I figure out the first one, is in order to swap to a posi-trac, do I have to replace anything else other than the carrier and gears?

Thanks!