06 xc90 drivetrain noise

[dalecxc90]

I'm getting some noise, a whirring sound, from the back of the car. It gets louder as the car speeds up and doesn't change when the tranny shifts. The AWD does not work (back wheels don't engage) but hasn't for several months. Does anyone have an idea what it could be? Hoping for a wheel bearing but afraid it could be worse. The carrier bearing seems to be ok and the wheels don't seem to have any play in them. 117k miles.
Thanks