Posi Additive?
just do a burn out and see the skids? 
Re: Posi Additive? If you have a TOrsen diff, it is not required (but sometimes used to quiet a "rusty screw" sound in a few vehicles per torsens FAQ's on their website). If you have an auburn diff, auburn says it will not affect the diff if you don't run it, it just may make noise. So, worst case, don't add it and if the rear is noisy, just add it to quiet it down. Or, many synthetic lubes are "Posi Compatible" and don't need extra additive....so there is not really a wrong answer here, you won't hurt anything either way.
how do I know if I have an auburn? 
Re: Posi Additive? If it is a torsen, lift the rear axle, put the car in gear and spin a tire, a torsen will spin the opposite wheel backwards...an auburn will not turn at all (if in gear). Or just buy gear lube with additive in it....
