The only way your going to find the problem is to yank the trans out, open it up and stare at the PDF exploded views located on the Traxas web site to make sure everything is in working order.

It could be tons of things. Stripped out gear in the priamry shaft gear, grub screw pin came out and stripped out a tooth on a gear, ETC ETC.

Trust me, your going to be in that trans multiple times if you are thinking about keeping it for a while, might as well open it up and get yourself famular with it, make friends with it because chances are you will be spending alot of time inside of it lol...