Check the pressure in your spare tire. The tire is mounted by pulling it up against the bottom of the car with a cable. If it deflates over time, then it gets loose and hanging from a cable can result in quite a vibration.I have a noise in the rear, driver-side 2nd row seat. While driving, the seat is just shaking like crazy laterally and making a very annoying rattle sound. When the seat back is folded down, the noise goes away. When someone sits in the seat the noise goes away. Talked to the dealer and said I would have them look at it on my first oil change. I've also noticed the folding armrest can tilt lower than the passenger-side seat. Not sure if this has anything to do with, maybe just needs realigned.
If this isn't it, how are your tires?
Your seats aren't the problem, they are the symptom telling you that something is causing a vibration.