[Al Skierkiewicz]

Justin,
You would think that the motors you have chosen would not back drive but in the configuration you are using them, once they are moving the arm weight will continue the motion. Even if you set the "brake" jumper on your victors, it might not help. Some of the suggestions above will help the situation, I am a big fan of surgical tubing but it might not be suitable for you design. Changing the ratio of the output shaft to arm might also help. I think a combination of a few ideas will solve your problem.