[JoshJ]

For simplicity, like its been said, you can just link each side with chain and sprockets or gears, etc

For driveability, i would use a gyro to give you steering feedback and have it adjust ur drives(not that i have any experience with them, but that seems to be the general trend around here)

For traction, maybe you could use a non powered wheel with an encoder on each driving side. That way, if a wheel slips, you can slow it down to the speed of the non powered wheel, (or just above it, depending) until the two speeds match, then you know that you are getting grip, and are good to go. Let me know what you think

-Josh