Off the top of my head, I can think of a few ways to drive the casters without transmitting steering torque:
- Slip rings. Models designed for windmills should be adequate and reasonably inexpensive (discarded)
- Counter-rotation (already discussed)
- Battery AND Motor on the caster module (control wirelessly)
- Pneumatic rotary actuators. Run the air in through a swivel down the axis. This option may also save you weight on gearing, and is insensitive to stall/low speeds. I've never done anything like this, just an idea.
If FIRST isn't your life, then you're doing something wrong! - BandChick [more] 
Hybrid Mode
