I've had the exact same thing happen. Cleaned out the diff and it was still gritty after re-assembling it. I'll have to check that clip to sand it or just replace it to see if it helps. This is good insight, thanks for sharing.
This is my theory.
My diff worked fine on its first outing. Before using it I pulled it apart and put more grease on the main gear balls. Set it and it worked fine. After this race meeting, I pulled it apart and cleaned it all. I had trouble getting the c-clip out. I was using a set of pliers that I had filed down into a point to be able to close the c-clip. Once cleaned, re greased and back together it felt like ‘shit’. It was rough when turned, I had to tighten it all the way up just so the main gear would not slip, but then it felt like a spool. What have I done?
I sanded the c-clip to make sure there were no burs on it. A little better.
So I order a new bag of c-clip and the Hudy c-clip pliers.
Installed the new c-clip and the diff was as good as new.
It was like the c-clip was squashed or dirt/grease in the groove and the c-clip wasn’t seated correctly.
Try a new c-clip if you haven"t already. That's how I fixed mine.