I have a chevy 12 bolt on my 55.2 (that came off a '78 blazer). I'm going to pull the rear differential and put in new fluid (I don't think it's been done since ...1978). I know you need to put additive in the oil if it's a posi rear end, but I don't know how to tell. There is no tag on the rear end. I did the test of spinning one wheel, and the other wheel spins the opposite way, but some of the reading I've done says that is NOT 100% accurate. Any one have tips on how to tell if the rear end is posi or not?
