Do you have any accessories on the car - like a roof vent / spoiler / roof rack? I know if you open a sunroof it'll vibrate, and assume somewhere along the way another item like the above may be able to cause a vibration too.

It's possible some mount is missing on part of the under-car plastic, allowing it to vibrate instead of being held in place. A friend driving next to you may be able to spot a tire wobbling, plastic waving around or something like that.

In larger vehicles (station wagon) or something with a weaker frame torsionally, there can be sidewall variation in the tires. To see or fix this, use a road force balancer - that applies force to the tire while being balanced. Though if that was the case you'd have felt it in the steering before or after the tires were rotated.

There are weights on the driveshafts - one could be off balance... though I figure that would come through the steering too.

Have you verified this remains vibrating when in neutral? There could be something like a crankshaft out of balance. rev the engine to highway speeds and see if it vibrates.