I guess it's a silly question, but you say; "could only hear it with engine off on way to junk yard to pick one up..." Were you being pushed/pulled? Not being a smart @$$ but I am having same issue and want to understand this troubleshooting process.
I have been to the wild vents thread, when I was first having the issue with my ac going out on acceleration. I replaced one of the of the only two check valves that I could find, which helped for a while. Then the ability to change the air flow AND no more cruise control cropped up.
The other question; how many of those check valves should I find on a 1998 5.9L? I replaced the one just off of the passenger-side intake manifold. There is one other just above that that passed the blow/suck test, no others to be found, I traced my vacuum lines around.
*EDIT - I looked at the location where others reported finding a check valve; under the battery and nothing. The vacuum line appears to have a splice where they used a piece of wire shrink wrap? Could this be the issue? Should there be a check valve there? The cruise control worked prior to the first check valve being replaced.

Thanks gang!