second that, check the drive cups 1st.

if they are good, then it could be the rear diff. but usually, if a diff has gone bad, you can hear it. clicking or grinding sounds.

check the rear tires....make sure the hex's and drive pins are there. even if only 1 tire was missing the drive pin....or the wheel hex was stripped...the diff would "unload" to that wheel and the power wouldnt make it to the ground.

there is a hardened steel diff unit. redcat is currently out of stock on them, but you may be able to find them in stock else where.

part # BS803-025A

also, i do recommend replacing the gear case as well. you can buy the gears themselves seperatly, but since the case is plastic and the screws that hold them together can get loose, its best to replace the case at the same time. the case is important, since it holds the ball bearings in place. if those have any play, then the gear's mesh will have play and your new gears wont last long.