[Driven5]

I know you said it's felt more than heard, but here's something I would still consider trying anyway: Download a free frequency spectrum analyzer type app (like Spectroid) to your phone, and see if you can get the vibration to show up. If it does, get it do so in at least two different gears. Record the gear, engine speed, vehicle speed, and vibration frequency each time. This would allow you to narrow down your list of potential culprits. It sounds like we can be pretty confident that it's not before the transmission output shaft/bearing, but this should also be able to tell you if it's before or after the differential.