I don't know when they stopped using vacuum shift I have how ever found them in trucks into the mid seventies. My 56 and 64 even have the same rear ends. The vacuum switch however is different, the 56 uses vacuum to shift both the rear-end and the speedo, the 64 is vacuum for the rear but eclectic for the speedo.
I have found a degree of difficulty in finding vacuum shift parts and am thinking about updating the axles in my 64.If I do this I will most likely use both the front and rear axles from a mid to late seventies gas powered truck with the regular 11 inch 10 lug bud pattern. I would also like to do this with the 56 how ever that takes narrowing a front axle but thanx to Grigg I know that it can be done
. If this is a truck you use on a regular bases and need the reliability I would recommend the change to something with better parts support even new truck brake and down time can be a killer if you are waiting forever for parts.
