2001 toyota sienna vibration?

I have owned a 2001 Toyota Sienna minivan for several years now, and we have sometimes had a vibration felt strongly in the steering wheel. It’s a problem that often occurs at the worst time – When we are on a long trip! The tires are good and balanced, and have been replaced, and balanced several times without really changing the problem. The struts/shocks have been replaced. There is no noise ociated with the vibration, nor is there ever any strange noises. The vibration happens intermittently. I have a theory that it’s related to the rear brakes, because once when we had to have the rear rotors replaced, we noticed the vibrations were worse than ever. The mechanic said that the new rotors had been very hot, and that he adjusted the rear brakes out some, and the problem went away. That makes me think that they are auto adjusting to be too tight, or that could be just a wild goose chase. It could be bearings… When I pull over after vibration the front wheels are often very hot, but I don’t know if that’s normal. I’d like to take a vacation this summer, but relying on the Sienna scares me. Other than that problem, it’s been a great car. Any ideas?