http://www.rc-monster.com/proddetail...adapter&cat=40 - 2 speed adapter
http://www.rc-monster.com/proddetail...ears1st&cat=40 - 1st speed
http://www.rc-monster.com/proddetail...peed2nd&cat=40 - 2nd speed
With the above parts and a brake servo mount(I have one, though it doesn't currently show on the site), you can maintain the 2 speed and run mechanical brakes. You could also drop in a center diff and use esc braking if you wanted to simplify the conversion. :)