Actually, I just now see that those wheels look like they're on axles. That may not be way what I want. Specifically, I want to use differential drive, as outlined in Admin's tutorial, except I don't want to use casters for my front two wheels. I want to gear the right back wheel to the right front wheel, and the left back wheel to the left front wheel:
Left Back Wheel XXXXXX Motor Shaft XXXXXX Left Front Wheel
Right Back Wheel XXXXXX Motor Shaft XXXXXX Right Front Wheel
I'm new to mechanical engineering stuff, so this isn't trivial for me. My motor shafts have a keyshaft. I guess what I need is a gear with a notch sticking out in the center to lock onto the motor shaft, then have a set of gears on shafts to link up with the the right front wheel. Those gears won't have notches, and those shafts won't rotate - only the gears. That chain of gears will connect to a gear on the front wheel shaft - that shaft will be able to rotate. The gear on that rotatable shaft will also have a notch in it I guess. Likewise with the right back wheel. And a mirror image on the other side.