I remember back in the 70's there was an additive available for positraction rear axles. Is this still necessary, and if so, where can it be purchased?

It still exists. Dealer, Amazon and many others sell it. Some synthetic gear oil contains it already (I think Royal Purple and Lucas?)

It's added to prevent chattering clutch plates in the posi unit when turning.

Can be purchased from a GM dealer under part #88900330.

Thank you gentlemen!

I found that for some reason the only stuff that works is the GM additive, I don't know the exact reason as to why the other products do not work well. Every time I used them I had to remove the fluid and go back tot he GM additive, the clutches were locking up

The GM brand is the only one that works.

My experience is the same. 1971 K 10 with limited slip differential. Nothing worked except GM additive with GM lube.
I always heard it was whale oil. Can you comment, Gene?

Mike

The original additive was made from Sperm Wale Oil. At one point the Sperm Whale became off limits and a new formula was made. This was probably thirty or forty years ago.

I've had success with the GM product as well.

Same experience dude!