[Mark McLeod]

I don't expect encoders to be terribly reliable going over the rough defenses.

To match speeds you could use a PID vi to modify the motor speeds on each side to track a desired speed.
In other words, speed up a motor if it's going slower and slow down a motor when it's going too fast. Or let one motor run according to the joystick setting and have the other motor match speed with joystick input modified by a PID check.

I'd suggest playing with PID on a standalone motor first to let you get the hang of what it is doing and how it works.

Do a search on PID in the whitepapers here on CD to start with the basic concepts.