I finally put an end to this saga. Here's the rundown:

1. Shop 1, Mar '06: said it's the tranny. It would have had to be on the output shaft, since the noise was exclusively speed related.
2. Shop 2, Deltech transmission Apr06 or so: Actually did their homework and put chassis ears on the car while driving it around. They isolated it to the wheel bearings. They get a high rating for their honesty and good work. Unfortunately, they don't do rear wheel bearings.
3. Shop 3, MB Wilmington, Dec 06: Also diagnosed it as the wheel bearings, and volunteered to do it for about twice book rate. You have to admire them for being able to say that with a straight face.
4. Shop 4, Autobahn Automotive in Lakewood, New Jersey: they actually did the work. Dave, the owner, is on the forum here, and I'd rather give him my money than a local of unknown talent. He did the work in a day as promised and charged book rate. The roar is gone!

I don't have a clue what would make the bearings become loud like that. It never got worse in the year that I drove it, and was always the same on turns or going straight. It just sounded like the droone of a multi-engine propeller plane.
I have to think the damage was done by the shipment, but I can only guess how. When they ratchet the car down with the hold-down chains, it must have somehow put a bad load on the bearings, which caused some sort of damage during the 1000+ mile transport.