New guy here. Found this thread in a google search, I have exactly the same problem down to the T with my '85. I haven't going from 2H to 4H then back yet. Just picked it up, btw. Check my sig for more pics, its got 92k and it was $900. I'll have to check that vac switch. This beast is a whole new adventure for me, I've only ever driven RWD Volvos before this. Love the truck other than this issue, glad to see it appears simple!