That's a small gearbox that changes the speed of the speedometer cable so the instrument panel reflects the actual speed when switching from high to low ratio in the rear end. It should have either a mechanical linkage or maybe a solenoid to make the ratio swap as the selector is moved from high to low and back.
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!