Shimmimg is the only way to extend the period between diff rebuilds. I think its 8mm and 10mm shims. I got the Yokomo brand. They are quiet cheap. I looked at tower and rcboyz but could not find what you need. I am sure someone else will find a link for you and post it.
My front diff required 0.6mm of shimming on the crown gear and 0.3mm on the pinion. That is a lot of slop or lash. There is no wonder i was going through so many gear sets... Better now though.