I've found that AW Grease is great for packing inside the thrust bearing of the ball diffs. My thrust bearings seem to last forever (I don't mod race though) as I've never had to replace one. I do build my CVD's and suspension balls with the stuff, and simply wipe away the dirt with a swab as it collects. No biggie.
In non-race applications it's great to coat the diff bevel-gears in cheaper cars to give a little limited-slip action. This really transformed the way my old Hotshot put power to the ground...