Turn a small 3 speed like a Chevy passenger car torque tube trans backwards and bracket it into the driveline behind the main trans. Use an electric solenoid or a vacuum setup to shift it between 2nd. and 3rd gear. 2nd. will be OD and 3rd. will be direct drive. Both gears will be synchronized.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!