i also have done something similar but only in nitro trucks rather electric i would almost garuntee that it is your rear differencial i went threw my front diff. then checked all threw my trans then finally check the rear diff. an that was my problem the whole time i stripped every flucking tooth off of my big gear inside luckly i had recently bought a big buld part lot off of a guy so i had a few extra to play with. but yeqah check the diffs first may seem easier to do the tranny first but trust me it aint lol