Try this.. Have your wife lay on the rear floor, and put her ear on the driveshaft hump right behind the console.. If it's the center bearing, the noise will be very loud, and you will be able to feel it at that point..
If a tire is bad on the front, you will feel it in the steering wheel.. If you think have a bad tire on the rear, move it to the front.. If you don't feel it in the steering, it's probably not the tire..
On mine, the Center bearing made a low pitch roar/vibration at 35-70 mph coatsing, or under load and became less noticable at speeds above 80.
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