Anyone have problem with their posi chattering?

[Road King]

I just bought a 68 that the posi chattered. I figured the previous owner had old/bad diff fluid plus I wanted to see what ratio it had so I pulled the cover. I put in 80/90 along with GM's additive and went for a test drive. The rear end was very unpredictable, it would lock on turns and be open on a few straight line burnouts. I noticed a few posted to do 10 doughnuts left and right. Do you think that will solve my issues or is some mechanical work needed?
Thanks