HI all, new to this forum (and RC in general). My son has an ECX AMP dune buggy. Lately, he has had an issue where one rear wheel will spin when the throttle is pressed, but the other wheel spins slowly or not at all. When I turn one wheel backwards by hand (no throttle) the opposite wheel spins in the opposite direction, which I gather is what is supposed to happen. I don't know much about these, but I can take stuff apart and get replacement parts. Could it be the differential? Or some sort of seized bearing on the non-spinning wheel? Any advice greatly appreciated.