Check the cable that runs from the push pull button on the shifter and runs down through the floor underneath the truck. I had mine separate under the truck that was allowing the whole thing to move instead of just the inner wire of the cable. I bent up a piece of metal with a series of small cable hold down clamps on it to hold the outer jacket of the cable in place. Works great and has shifted this way for  quite a few years now. Mine would shift into high but would not go back into low without crawling underneath and holding the outer sheath and moving the inner wire on the shift cable. Hard to describe  the clamp I made and it took a while for me to find exactly what was going on but I fixed the problem with the new bracket and hold down clips.

Thought I had a picture somewhere of this but can't find it at the moment.

By the way mine is a 1978 with vacuum 2 speed shift as well.