Well, it is most likely the radio/ control board on your tank. If a track moves in one direction and not the other likely something failed in the electronics.
These tanks are pretty good but the radio control systems are just plain cheap. Probably all of us here have seem failures. Nearly all of us remove the stock electronics and replace them with a new control board and a real hobby grade radio.
i dont have any other advise for you other than to search for a new control system on ebay or from a supplier unless you want to upgrade to hobby standard stuff which costs about 150- 200 bucks minimum.
if you can wait, i will soon be gutting my abrams for an aftermarket system and i can send you my stock system.
are you able to unscrew the tank hull and unplug the old system and plug the new one in yourself?