You have no idea how happy I am to know that I'm not going crazy and imagining the vibration at 50-60. Mine does the exact same thing, and this is my second SS (admittedly, of the same design).
I also went back to the stock rubber bushing in the link directly under the shifter, and it did not fix the humming.
I truly love my P5, but the two worst parts about driving and owning it have hands down been this short shifter vibrations, and the absolutely horrible, jerky, constant-vibration engine mount situation I have going on. (Also my warped flywheel, but that's another issue.)
I am EXTREMELY interested to know if you're able to fix things or make them better. I wish I had more time/experience to experiment with these sorts of things, but thus far my experiments with motor mounts have ended in more drivetrain and cabin vibrations, and this is also my daily driver, so never enough time to have it on jackstands to work on it.
Keep me up to date for sure. I love learning about these things, and would be tickled if we could figure out how to improve these sorts of issues.
Yup - going to do some experimenting on the engine support crossmember this weekend (especially that rear single bolt/nut mounting point) while I have the car jacked up doing other front end stuff and will provide an update if I find something that helps (or not).
It's also possible that the motor mounts may have not been tightened and torqued per the service manual's sequence or some other engine/transmission/exhaust support system issue going on.
Question - Along with the 50-60mph shifter vibration, does your vent system blower fan seem noisy to you? Not the leaves and trash getting in it kind of noisy, but the blower fan noise and the shifter vibration seem to harmonize and amplify each other at that speed.
There are times I will turn the blower fan off to listen/feel for the shifter vibration at 50-60mph and the shifter actually seems to be much quieter/ok with the blower fan turned off and the blower fan actually sounds to be more the culprit.
Just wondering if all P5's blower fans are noisy with mediocre air output or maybe just dry/wore out shaft bearings from age in my (son's) P5?