There are several possibilities; off the top of my head, they include:
1. Out of balance wheels -- maybe a balance weight came off, or something is caught inside a wheel. I had a badly out of balance wheel once on my motorhome that I eventually discovered was caused by a bird (!) that somehow found its way into an inner rear wheel.
2. Out of round tires, not always immediately discernable by the eye
3. Brake dragging on a warped rotor
4. Out of balance drive shaft or worn drive shaft bearing/hanger or U-joint
5. If just starting out after being parked for quite some time, the tires will often be flat-spotted for the first few miles, which is nothing to be concerned with. This solves itself as they warm up, etc.
This is almost certainly not an exhaustive list!
For #3, there would be other pretty obvious effects: a wheel that's a lot warmer than the others, hot brake odor, the vehicle seeming to pull to one side when the brakes are applied, I think particularly if it's a front brake that is dragging.
For drive shaft problems, the speed that the vibrations occur at is often lower than for wheel problems--maybe 30-45 mph to start vs. 60+ mph.
