The driveshaft does not change speed. There is a planetary gear unit in the differential that changes the speed of both axle shafts when going from high to low range. "High" is the actual ring and pinion gear ratio, while "Low" reduces the axle shaft speed with planetary gears. As above, the unit is spring loaded into the Low range. Jacking up only one axle will double wheel speed with the differential gears, just like it does with a single speed rear end.
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!