OK. I replaced the idler arm and the car still vibrates rather violently at 40mph and up.
When I turn the steering wheel, the vibration stops. This is what the old idler arm looks like, versus the new. I don't think it was the problem. The steering damper is relatively new. Ball joints seem good. Could it be the drag link? It DOES have some up and down play, but I'm not sure if it's supposed to be that way.