May i direct you to the WhitePaper sections?
There are 2 drive assembly word documents that include some explinations and full cad prints. One is called " Dual Motor Gear Switching Assy" and the other one is called "Gear Switching and Motor Mount Design"
Also notice that the first one uses a fisher price motor that is "piggy backed" to a drill motor (not an easy task to accomplish, but it works great) and the other one only uses one drill motor, both include gear switching.....The "Gear Switching and Motor Mount Design" has been out for acouple years and has been tested and used by numerous teams....
Hope this helped...
(EDIT: You can also get all of the above at
http://www.technokats.org/robot.html )