Probably the best advice I can give is to go search through the sbv2 thread. There is a lot of good info, but you may have to dig a little.
Academy's new 1/10 scale buggy SBV2 PRO
There is a detailed walk-through of building a smooth Academy diff on this page from the Gv2 thread.
New ACADEMY 1/10 scale 2WD BUGGY
Also, do you have the metal outdrives or the plastic ones? The plastic ones are notorious for causing problems. I prefer using associated diff grease as opposed to the kit stuff; it seems to last longer and make the diff action smoother. The slipper should slip before the diff, and the diff should not be run loose or the rings will groove really fast.