Hey, I'm all over this one!!
This is evidence a cyclic resonance harmonic vibration between two vehicle components.
They had a similar problem with the F-32 fighter. Their problem was when both engines were at the exact RPM there was no vibration. However when one or the other of the engines oscillated in RPM, which one usually did, the vibration would occur. Thus vibration, no vibration then vibration over and over again until they dropped below Mach I.
Your solution would be to weld the rear differential so that both wheels on always rotated at the same exact RPM.
If you buy into this you're as goofy as me.
Oh, I just dreamed up that F-32 stuff too.
However, I would give rotating those tires around and see what happens.
Those kinds of things drive a fella nuts.
I have a "click" showing up in the left front of my 01 Chev 1500. The farther away from home the more I think about it.
Keep lookin' you'll find it....