Yep vacuum shift was the standard way of doing it on these trucks, I'll bet it
Looked like this, right? Check the lines for leaks and the vacuum valve uder the cab, also there will be some vacuum lines under the hood that feed to this setup. You may want to see if it is making vacuum. My guess would be in the line from the switch valve to the diafram in the hight side, it pul;led back into low Ok.
To test the high gear in the axle, remove the vacuum diafram from the rear axle and shift it manualy, if it still jumps in high gear, then the gear set is bad.
Or I just though of this, switch the hoses at the axle and see if high gear works now, and low is messed up.
