I am getting a slight shake between 70-80 mph and steering wheel wobble. I am leaning towards the axle half shafts on the front. If its happening under load, on the gas, its definitely a cv joint. You may hear clicking too. These are easy to replace. If you mechanic is talking about the driveshaft to the rear wheels thats not the problem.
What I noticed on my rx300 was that after a 75 mile highway run, I put my hand on the hub of the wheels and all were warm, not hot. But after driving in local traffic for 1/2 hour and rechecking the wheels, my passenger front wheel was hot enough that I couldnt keep my hand on it. The drivers side was warm. So if it was bearing, it would have been hot from the highway drive and if it were the brakes, the calipers would have been hot too, and they were cool. I can assume at this point that the passenger halfshaft is failing. Combined with the inner joint grease slung up in the wheelwell, I believe I found my problem. Hopefully this will help others diagnose there vehicles.