Do you mean the idler gear? (Gear between the diff gear and drive-shaft)... if so, they are prone to break, especially if you run a tigh setup. a) Slipper gear overtightened, the slipper is the transmission saver. If you run with a tight slipper you are prone to bust the tranny b) tightened diff gear, duratrax states an adjustment of snug-1/8 of a turn.. make sure that by snug you are no tightening it.

finally, sometimes plastic gears loose roundness.. take the tranny apart, but the leave the 3 tranny gears in place
... manually turn the shaft gear..is it smooth? does it have any rough spots? lack of smoothnes is a bad gear.. rough spots is usually a gear with obstructed teeth (i.e. small rock)...

If all else fails. Go metal. I got a metal idler and diff gear with carbide balls. Never worry about the tranny anymore. I read a post once here that some1 had a bad experience with metal gears.

Happy troubleshooting.