So far, every time I think I nailed down the cause, I proved to be wrong after another day or two of driving... but this time I really think I have it!
The other day I noticed the vibration noise when I shut the driver's side door - a golden opportunity to try to track it down, since I could replicate it without having to drive around and aim for rough roads.
So after some repeated door closing, I verified for sure that the noise was coming from the center of the rear seat, as suspected. So I climbed in back and tapped my fist around on top of the center of the rear seat until I heard the exact same noise that's been bugging me for weeks. It was clearly coming from the gap in between the seats... but as anyone who's already checked this area can attest, there's nothing in there that should be making this kind of noise!!
Well that's just it! Even though the noise absolutely sounds like a plastic-on-plastic (or maybe plastic-on-metal) vibration, that not what it is... it's actually the sound of the two leather seats rubbing together as the seats vibrate slightly. I'd have never believed that they could make the sound I was hearing until I watched it happen 6 inches in front of my own two eyes!
With one hand tapping on top of the seat, producing the noise, I could stick a finger into the gap between the seats and it would go away entirely... it's as though the way the seats come together, they form some sort of acoustic resonance chamber that picks up some slight vibration and magnifies it off the leather pieces rubbing together... or whatever
(This, by the way, explains how lowering the armrest usually makes it go away completely, as when the armrest is up, it spreads the leather opening out a bit and effectively squeezes the seats a hair closer together.)
So now the next step is to attempt to verify my theory by driving around for a few days with a towel or something stuck in-between the seats, hopefully in blissful silence
for those of you with the same noise, give this a try and let me know if it works for you too.
Good luck!