Since the drive system is front-wheel drive, the foot leaning aft was changing the center of gravity. In plain english, not all the weight was on the front wheel. On some surfaces, R2 would just spin and struggle to drive forward.
However, the foot drives have an adjustable rear omni-ball caster...
...all I had to do was unscrew the onmi-ball until it leveled out the foot.
Now the little guy is all level again.
