separate the rear motor well from the main chassis, leaving only the motor wires connected to the motor. Use a flat narrow copper strip of metal, and reconnect the motor well/axle to the chassis, via the strip. Simply bend the strip to adjust height. The strips tension, depending on how far you choose to have the motor well/axle extend out, will provide the suspension you need.
Hope you can visualize this... Its one of our future mods, but not on the priority list.
www.questformadness.com
(the Bit Modifeds how-to page)