Well as I said, all of the motors DO work properly when the motors are not yet placed in the robot's chassis. That is, they are just on the table connected to the motor shield & Leonard. I've never touched the jumper - it's always been on the motor shield.
The problem occurs when I put the motors into the chassis. That's it. It's like putting them in there makes everything begin to malfunction off of the same DC power that I used when they were outside of it, but when I use USB, it all works fine.
If I didn't know any better, I'd say that placing the motors in the chassis short circuits them, but the fact that they will run on AC power, seems to negate this.
I'm stuck.