Here is my first go around with Sketchup. I realize it SUCKS, but I can't figure out that dang rotate tool. My thought is that a rack gear would be moved by a servo and move the gears in the way of the motor gear.
Here is the part that is up in the air. Since cone gears don't exist (or aren't easily attainable) I thought it would be more fun if the robot had to use a clutch to move a gear away and put it back onto these gears to make it shift. A cone gear would be smoother, but this will be more of a challenge ;) This project should get some more attention when my extra servos and rack gears come in the mail.