[KingLT1]

Agree with all mentioned above. These diffs are very sensitive to the type of fluid used. I could only get mine to stop making a "groaning" noise on turns with GM fluid and adding the posi additive. Even at that when it gets below 50 degrees out it still makes a slight noise on the first couple turns then it's dead silent. I consider this to be "Normal" for these rear ends based on research and some performance shops I have talked too.

So I would try changing it yourself with GM fluid / additive first(it's super easy), and if it doesn't get better, you will likely need to replace the diff.