How often is everyone rebuilding their diff?

[AaronR]

After 4 race nights (3 runs each... 2 qualifiers and 1 main) or 12 5-minute runs, my (rear especially) diffs sounded very gritty when spinning them by hand. No serious noise (like a chirp while landing after jumps) while on track either which I thought was weird too. I rebuilt (regreased basically, used same diff balls and rings as I saw no wear, just not much grease left) both diffs yesterday and they are much quieter but still seem gritty. I'm using Tamiya Ball Diff Grease which seems thinner than AE grease from what I remember. I can go 10-12, maybe more race nights in my B4.

My track is indoor off-road, hard packed moist dirt (no real dusty areas), so it is a fairly clean environment.

Any tips, comments, suggestions?