The smoothest diff I ever built used rings that looked like they had been run in a sand drag car for 20 years. Those things were GOUGED. Sanded them for about ten minutes (no hobby shops around, getting ready for a race in just a few days) and put it back together with fresh balls and a slapmaster. Smooth as butter. I'm still using those rings with just an occasional sanding.

I really don't understand how anal some people are about diffs. It's like it's become a competition for the smoothest, freest diff instead of the fastest car. Sometimes a stiff crunchy diff is actually better.