2003 Volkswagen Jetta Rear end vibration in 2003 Jetta

I have a rear end vibration. At low speeds (5 - 20 mph) it feels like the car is driving over a small bumps in the road evenly spaced, or as if there's a lump on the tire (but there isn't). At highway speeds there is a low volume roaring sound coming from the rear end (not unlike snow/studded tires might cause). Otherwise, the car runs well and there are no other symptoms. There is no vibration in the steering wheel.

My guess is one of the following:

- tire our of balance. There is no unusal wear on any of the tires.
- bent or damged rim. No visual evidence of this.
- rear wheel bearings require repacking or replacement.
- other.

Can you point me in the right direction?