Creating a transmitter-controlled 2-speed transmission would be pretty time-consuming and difficult.
You'd have to replace the electronics because you'd need a 3-channel receiver (and transmitter for that matter). Then you'd have to add another servo, or at least an actuator to work the shifting mechanism, not to mention the mechanical gear setup.
If you're dead-set on a 2-speed tranny, you're probably better off going with a centripetal clutch like the non-tx-controlled nitro cars have. When the gears spin fast enough, the 2nd gear engages and the 1st is allowed to float on a 1-way bearing. Some use a pin and spring type while some use a shoe and spring time (the shoe type offers smoother engagements).
But even then, you've got your work cut out for you.
