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Don't laugh, but it simply sounds like a bad transmission mount. You've done so much trying to chase this down, try removing the mount and see if the rubber has degraded. When it fails it allows a metal to metal contact which will create a harmonic at certain rpm levels, regardless whether the car is at cruise speed or sitting at the curb in neutral.





I chased a vibration in the same RPM range for a couple years in mine. Had the carrier replaced, u-joints, shaft balanced, balancer. Turned out to be an UNBROKEN, yet old and hard trans mount.