Hum from the drivetrain?

My car is humming starting at 35 mph, and it sounds like its coming from the rear end. The rear diff is full of fluid, as is the transmission. The hum occurs when I am cruising at a constant speed, it goes away as I accelerate and decelerate. It also goes away if I push the clutch in and coast... My local ford dealer said all my axle bearings are worn out and the rear end needs to be overhauled. If it were bearings in the rear end, wouldnt it hum constantly even when I have the clutch in coasting? Also, my car is '95 with 64,000 miles, would the bearings really wear out already assuming the car hasnt been too abused? Thanks for any input.

I also thought it was just the carrier bearing or pinion bearing, but you are right about replacing them all while its torn apart. I guess on the positive side, its a good time to throw in a new ring and pinion!