Well just tore open the rear diff and definitely some wear and tear... 3 of 4 of the small bevel gears are worn... Looks like small grooves starting in the middle of each gear... We could have a winner!!!

Always keep your diff's well oiled... I recommend you check this if you buy any RC second hand, even if the guy says he just refilled the diff's... I remember this for the next one (LMAO)...