Originally Posted by mikezap
normally i am tightening the diff down and backin it up a bit
I would go for an improvement right here. The best way to set a diff is somewhat different. Tighening it down and loosening it can easily put flat spots onto the balls/rings and you will never have a smooth diff. Most likely it will have to be run tighter than it should to avoid slipping and won't last as long.
First, when assembling the diff stop tightening it before you even feel resistence. It should be completely free. Spin it in your fingers for a few seconds to ensure everything aligned properly and smooth. Now tighten it just to the point where the balls and rings touch. Keep it light enough so you can even make it slip using just your fingers. Now is the really critical part. This is where you begin to "break in" the diff. I usually hold one outdrive and spin it by rolling it back and forth (usually on my arm but I know a few guys who took and old dogbone, cut it off, and chuck it into a dremel). Do this for about a full minute, then tighten it slightly (~1/8th turn) and repeat 1 or 2 more times before installing it in the car.
Now with it in the car it is still probably too loose. What you want to do now is set your slipper to your preferred setting (usually by # of turns out) and hold the right rear tire and spur in one hand and try to turn the left rear tire with the other. Watch the slipper shaft to see if it turns before the diff slips. If the diff slips first, tighten slightly and spin it for another minute before testing again. Do this until it is tight enough so the slipper slips first, and the diff is nice and smooth.
Using this method should seat the diff enough that you won't need to tighten it again after the first few runs. But still make sure it is not slipping before the slipper from time to time.
It sounds very time consuming, but once you master this practice it goes by very quickly. I can completely rebuild the diff in my Losi gas truck (which has to be torn almost completely apart to get to the diff) in less than 30 minutes and be ready to run a 30 minute main without worrying about the diff loosening up or letting go.