[AdamHeard]

Thinking about it, I think it will be impossible to accurately "tune" the ball differential.

If you put it on blocks, the load on the wheel will be substantially different than when on the floor, and if it looks like it's working, it most likely won't when you put it on the floor.

If you try to tune it on the floor, the floor will make it appear to match the speed of the outer two wheels, regardless of whether or not it actually contributes.