You did add 2 bottles of friction modifier, right? One won't do it.
In the old days, I had two Chevelles that had the 12 bolt differential with an Eaton posi-traction unit and I liked them a lot. The 65 Chevelle I bought new and put 100k miles on it, much of it 1/4 mile at a time. At about 70k miles, the clutches started popping and snapping in the corners. Changing the fluid fixed the problem. At that time, 1968, the lubricant was sold at the dealer in quart cans and it had the additive already mixed in.
The other one was a 69 SS396 Chevelle that I bought used and kept it for one year. During that time I changed the rear gears from 3.73 to 3.31 myself. I had a time getting the two plates withe the 4 springs back in place between the clutches, until I pressed them together with c-clamps and tied the plates together with fishing line (many loops) and then it was easy to install them and cut the fishing line and pull it out.
In my experience, the Eatons are great. And, the traction lock in my truck has performed well. I don't know why they decide they are too tight after 50k to 70k miles and need shims removed.