[BBray_T1296]

How about a variable diameter pulley system? seems pretty simple, and could be done with a simple Vex motor. We have one in our drill press. Just a turn of a knob and it varies by a spread of ~3x.

Basically you can relate it to a dog gear and receiver, except they always touch (the teeth are long), and are two sides of a V pulley. By pushing/pulling the sides together on one side of the two pulleys (that the belt runs between), the running diameter of the belt is increased or decreased. The other pulley has a spring attempting to lightly compress it, maintaining tension in the belt. As the motorized pulley expands, it demands more belt length and thus reduces the diameter of the spring pulley.

In ASCII terms:

|\....../|......|\..../|.....|\B/|
|.\..../.|......|.\B/.|.....|./\.|
|..\B/..|......|../\..|.....|/..\|
|=====-->====-->====
|../B\..|......|..\/..|.....|\../|
|./....\.|......|./B\.|.....|.\/.|
|/......\|......|/....\|.....|/B\|

The problem with this is it can only shift on the fly. It is unrealistic to shift while not moving, as it would be very difficult to pinch the belt upwards, so, in other words, not an FRC robot. It is a cool idea though