Toyota Repair: Previa rear end roar, pinion bearings, axle bearings

Question
92 Previa D, 216,000 miles, automatic transmission, 2 WD.
On a smooth road at speeds exceeding 35 mph. a roaring sound is heard from the rear...not unlike what my husbands big tread off road tires sound like. I have rotated the tires, replaced two tires, had the exhaust system checked. A friend sat in the back seat and had me swerve and put the car in neutral while going 45 mph and nothing alters the sound. It seems to be directly related to speed. No one can tell if it comes more from the right or left, from the middle or extreme rear of the van. I'm about to make a long business trip.....and don't want to get stranded on the road! I sincerely appreciate any insight you may have about my situation. Thanks!

Answer
Do not make a long trip under this condition, it sounds like the pinion bearings in the differential are failing, take it to a Toyota dealer and have them remove the drain plug and check for metal particles, the only other possibility may be that one of the axle bearings is causing the noise, but from how you are describing the condition it sounds like it is in the differential.