All three of the trucks I have with two speed rears are vacuum shift but they are not on the dash the valves are under the truck next to the transmission and controlled with a knob and cable on the shifter. Both the 56 and the 57 shift the speedometer with vacuum the 64 shifts the speedometer electrically so its knob and cable also has a switch built in for the speedometer. and my friend has a 72 that has the same rear end as my three trucks so the vacuum shifted rears were around till at least 72.
I haven’t seen the internals of the dash mounted valve but I bet you can rebuild it with standard o-rings and seals try and industrial outlet like Seal Source some of the bearing outlets also have a good selection of seals and o-rings. If you have a local industrial hydraulics shop you may check with them if they can’t help they should be able to direct you to someone who can.
A good Tech Tip
