Wow,,,, sounds like you have things pretty well covered, but still have the problem. All I can tell you is we had the same type of vibration in a 126. After all other possibilities were checked, I did a search on this site. I came up with a possibility of shiming the center driveshaft support a bit. I put a little over 1/8th shim under the support. That positively took care of the vibration. Hope this helps.

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