Just a thought, Working on vibes and acoustics on aircraft interiors has taught me the vibration source can migrate to any solid or semi-solid mounted structure and be difficult to diagnose. If you have any torque straps on the engine, or trans, try removing them for test. any solidly mounted objects like exhaust, linkages, engine mounts and trans mounts can translate vibes pretty easily.
I don't have a torque strap anymore. I did have one back in the 80's and took it off because of the vibration. I did go to urethane mounts on the engine and transmission, so that is probably going to do about the same thing.
