the axles only shake at certain speeds and since they are rotating slower then the axle is rotating slower which would put the shaking at a higher speed when running the slower spinning 16" wheels. I would like to see shaking speeds vs. rim configurations and that might help narrow down where to look.

but the main thing I was getting at is just check the axle shafts and see how much sideways play they have. the difference between my old axle and my new axle is amazing, the old one would move about an inch or so (and made a nasty vibration that started at 45mph and stopped above 55mph) and the new axle only moved a few millimeters, maybe 5 or less.

also checking the runout on your tires is important, my friend changed the rear tire on his R1 for a tire he got for cheap (like 50% off) and it had a .93" runout, slow speeds everything was cool, 55-65mph it shook like crazy and then above 65mph it settled back out. the reason for it only shaking at certain speeds is because it is at the resonant frequency of the tire (the frequency where any tiny shakes are exaggerated so to speak)

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'81 300D - swapped to a 617.951 turbo motor with a 150 amp alt.