[Al Skierkiewicz]

Jacob,
Even though you said each motor would run on it's own battery and speed controller, it sounds like the speed controllers will get their PWM from the same robot controller. If this is the case then they will find a common ground through the controller and that sounds real bad.
Use one of the above mentioned ideas for isolating the motor frame from the device frame. If possible, since the motors seem inexpensive, they may be easily modified to cut the connection to case. One of the users comments mentioned that ground is already on one pin of the connector.