Read my first reply again and do it that way. It's the same whether you have a sure-grip (a posi is a Chevy term. Both are the same operating characteristics) or an open.
sorry im still really new to this (haha im only 17) and when i get it up on the jackstands only one wheel spins while i turn the driveshaft. so im not sure how im supposed to make sure both wheels turn if only 1 will.... (sry i know this is prolly a stupid question but i really dont know)