sudden vibration problem

The day before yesterday I pulled onto a freeway I drive every day, and was surprised at a powerful vibration that came right through the steering wheel -- it visibly shook -- as I drove over a rough patch of concrete on the onramp, at about 50 mph. It happened again later during the drive .... and didn't magically go away yesterday. Especially if the car is making a sweeping turn at a fair rate of speed, it almost feels like I could lose control if I weren't gripping the steering wheel. There's no vibration on smooth surfaces, and none simply from making a turn. No strange noises, no other symptoms.

I'd think if it were something simple like losing a wheel weight, the vibration would be obvious at all speeds, on all surfaces.

And it's even stranger in that it began happening so abruptly.

Where to start?