I have an '81 Z28 and want to know if it has positraction. How can I tell by looking at the rear end? Will the vin or build sheet tell me anything about the rearend? If it's not a posi rear, can I build it so that it is?

jack up the rear and turn one of the wheels by hand, if the opposite side wheel spins in the same direction as you are turning, you have posi. if it spins the opposite direction, you do not.

Sound logical. Now if I don't what do I need to do to make it a posi?

a posi carrier. preferably the same series as what you have now if it's not.

Isnt it more fun to just smoke the hell out of the back tires and see if both marks are the same length?