I've been dealing with this issue for about a year on my nitro evader. If there isn't 100% traction (like on the street or wet dirt/mud), the car spins out under ANY type of throttle. I can get it going walking speed, and then slowwwly ease into the throttle, and the back end just slides out. I've tightened the diff down all the way, but still cannot for the life of me keep the stupid car straight. I know tires can play into this, but this is more than just that; it's like i'm driving on ice.. literally. I also own an rc10gt and do not have this problem at all.. in fact to put it in perspective, the traction of my rc10gt with road tires in dirt is better than the evader with proper dirt tires in the dirt!

So here's my idea. I rebuilt the tranny awhile back on the Evader, however I packed the ball diff/diff case with differential grease. I didn't just put a little, I really packed it in there. Is that the proper thing to do in these type of ball diff; could that be the reason I can't get that diff to lock up? (assuming the lack of lock is the cause of my straight line traction issues).