Please don't jack the car up on one side, start it up and put it in gear to see it it is a posi unit. If it is a posi rear it may drive right off of the jack into who knows what. So, jack up one side, put it in neutral, rotate the jacked up wheel by hand and if it moves easily it in not a posi rear.

I'm assuming this is what Sprint OHC 6 ment in his post but I thought I would clarify things a little.