My dad worked at Chevy dealerships as a mechanic for years. I bought an old 68 Impala that had a posi rear in it, and when I drove it home, I thought the rear end was going to blow apart with each turn it clacked and hopped so bad. Dad had some small bottles of GM posi rear end additive. He said he stocked up on it just before they reformulated it. The original was made from sperm whale oil. No kidding. When we took the cover off the 12 bolt posi everything looked OK, so I just changed the oil and put in the 3 oz bottle of additive. Presto...no more noise. I put 120K miles on that car and it got me through college.
