Basicaly, the smoother the surface you have, the easier it is for the balls to slip over the diff plate, when you 'scuff' the plate, it gives the balls more to grip to, so you can run the diff a little looser without the diff slipping, this also helps with wear on the plates, as the balls aren't being pushed into the plates.
The other big killer is to much grease on there, should be just enough to lubricate the balls, and no more, the thrust race, pack the bugger.
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