Finally got around to doing the toothpaste thing to a set of gearboxes, After two hours and 1/2 a bottle of Crud, they freewheel now! Funny thing though, about an hour into it one of the gearboxes froze up and the motor got really hot. I guess I caught it in time, sprayed those cans of air on it to cool it off, applied some more toothpaste changed the polarity to reverse the motors and away she went!

Thanks guys! 1 set down, 11 more to go!

BTW I hooked both motors directly to a 7.2v battery.