lol steve, not content with your own answers thread? haha :P
Diff rebuilds are something I tend to either not do for ages, or once or twice a month depending on any issues i'm having, how much i race or even at what track i race at.
Steve's answer, as always, was very spot on
As for the lube, i use anti-wear grease on the thrust bearing (really pack that stuff inside good and proper) and Team Associated Stealth Diff Lube for the balls and plates.
With normal steel balls and diff plates taken straight from the packet without prep I would suggest a diff rebuild once a month.
When I moved to ceramic bearings and ceramic diff balls I started to sand the diff plates with a 2000 grit before installing everything. Now these diffs just last for months without needing a rebuild, presuming the track you run on isnt dusty/sandy/etc. My theory being ceramic balls dont wear a groove into the diff plate as easily, but when they do its much smoother than the steel balls make and hence seems to last longer.