Leaving the diff alone and letting it run loose can end up doing a lot of damage to it and then youll be rebuilding it sooner than need be! If you tighten the diff as instructed above by amsterdamn you should be good to go! Running a diff too loose causes the balls to rub against the diff rings and overheat due to too much friction and the heat can melt the plastic diff gear! I know because I did that many times when I was first starting out in RC.