It's probably a bit late now, but I've just blogged a step-by-step guide to how I build diffs. I'm not any expert by any means, but this approach has given me a stock Ansmann diff that's lasted four race meetings and quite a lot of practice before needing a rebuild, and has just gone back together with all of the original parts intact and minimal signs of wear.
You can find the guide at
http://mrmagoo-rc.blogspot.com/2011/...ball-diff.html
Hope it's of help to you.