Moan at between 35-40 MPH

If it is changing as you drive it.... I would pull the pumpkin and check EVERYTHING. Preload on the carrier bearings, torque on the bearing caps, wear pattern and backlash, preload on the pinion bearings etc. If it has just started or is not quite set up right you can probably cure it. Once a gear set wears in to a noisy pattern it is usually there for the life of the gear set.