One thing to research if you are thinking of an aftermarket valve, the vacuum valve for my '57 has 4 ports, vacuum "in" from the engine, vacuum "out" for low range, vacuum "out" for high range, and exhaust "out" to the exhaust side of the system. The valve has to be able to shift the vacuum to the actuator in both positions but also has to be able to exhaust the circuit that is being de-energized.
I did some more research, and a "5/2 pneumatic valve" would do the trick. Those are designed for double-acting cyilinders, which the vacuum actuator on the two-speed axle would act like, and then have control over the exhaust. You would have the vacuum supply line go into the main supply port, and then tie the two exhaust ports together to go to the open-air vent. For the speedometer range box, you will have to tee off of the vacuum line that actuated the appropriate range on the axle.
I have attached a couple of pictures - one shows an example of a 5/2 valve, the other shows a annotated engineer symbol with the letters matching the ports seen on the example to demostrate how the valve would be hooked up. These valves are quite inexpensive - amazon.ca shows one for twenty bucks for a 12V version of the valve with 1/4npt ports.
Non-electric versions can also be found, but would require some creativity if you want it to be cable actuated like the original.
