no one makes a metal idle or diff gear for the rustler and i have mip cvds and have an OS .18 with a gallon through it and havent stripped a tranny gear yet (suprisingly) i dont know if the traxxas ball diff will matter because i used to only strip idle gears never the diff gear so you should be able to keep the stock diff, maybe your slipper clutch is too tight and needs to be loosened a little

im guessing the idle gear is stripped