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
