What diff fluid are you running in you e-truggy's. I have a Losi ver. 1 with stock fluid...way too much rear power. I think the center needs to go up but the fronts are already ballooning. I am guessing It came with 553 but do not know for sure. Also a aguess but 10-20-7 may be the ticket, please advise.
Maybe its your setup and not a diff issue cuz you are light in the center and rear already. When you say rear grip that is kind of a general term, is it too much side bite, or forward or both? Maybe you are lacking in front grip rather than too much rear. I would try going heavier in the front but be careful cuz this can really effect steering entering the corner off throttle, but will help on throttle, corner exit steering.
Take a look here too for some tips. http://www.rc411.com/pages/howto.php?howto=24&page=4