Its easy, you drive, turn the two speed knob to the left, lift your foot off the gas, count 1001,1002, and step back down on the gas. It should shift to high. You should feel a little tug when it shifts, and the engine rpms should drop slightly. Alot of times they stop working because of a vacumm leak, someplace in the steel lines running to the back end. If the holes a big one, then the engine will run rough at idle. Another place they go bad is at the bladder located on the diff case. This is a little harder to fix, but not impossible, its just a lot of little bolts that ussally break or have to be cut off. Once inside you find the rubber bladder, that decays over time. I've replaced it with a piece from a rubber boot(the thin kind you slip over a shoe) and had good luck with that. Also sometimes just the shft get gummed up and needs to be cleaned.
