Elusive vibration starting 55 and continuing to highway speeds

[spyro235]

Hey guys! I've got a driveline vibration that I'm really getting annoyed with. I thought it was the wheels/ old tires that came on the car, but then I switched to the summer 19's I got for the vehicle and what do you know, the vibration remains COMPLETELY the same.
If it makes any difference, I've got a base model 35D with NON air/SLS suspension in the rear.

A few things about this vibration:

1) It seems to lessen with more throttle, but when I let off at highway speeds, I believe it gets a bit worse. I will double check today on the way home.

2) I know it's not my wheels and tires. It's been the same with 2 sets of wheels and tires, completely different sets at that. 18's, then 19's, first with Michelin latitude snows and next with continental DWS.

3) The vibration changes base on what's in the car. If I have a decent load in the trunk, like 200-400 lbs, the vibration is lessened to a decent extent.

4) While towing, which normally puts 500-600lbs on my hitch, the vibratino is still present with just as much "amplitude"(felt vibration shake) but with a different frequency. It'll be smooth for about a second, then vibrate 3 time within a second, and oscillate like that.

My thoughts kind of lead me to a center support bearing, mostly because I'm not quite sure what the heck else it could be. It's clearly something that's rotating, and the slight change while in compression in the CV joints from a load/ squatting with the trailer makes sense to my engineer/track car mechanic mind. The vibration vibrated my passenger seat visibly, the back of the seat moving probably an inch front to back with vibration. I also commute 100 miles a day, 80% of which is on the highway, and I'd love to get this beast into smooth sailing mode. Any suggestions?