[RyanCahoon]

A couple of things come to mind:

- If the loading cord is above the ground (it's hard to tell from the video), there's a torque moment rocking the robot onto its back wheels. If the wheel that's slipping is in the front, that may be part of the reason why.

- (This may apply more later than now) Be careful you aren't applying too much power to the wheels. If the motor command is a step from 0 to 100%, there could be enough torque for the wheels to start slipping (regardless of the loading - simply because the rotational inertia of the wheel is probably less than the inertia of the entire robot), and then you're into dynamic friction which is (theoretically) less than the full static friction coefficient. Try ramping from 0 to 100% over a couple seconds.

- Have you tried an inclined plane test to determine coefficient of friction?