Could be bad bearings. With that kind of mileage I would inspect the wheel hub. Lift the vehicle and see if there is any play in the hubs by twisting on the wheels. Especially if they have never been changed. With the humming sound you describe. Its either the tires or the bearings! Also while its lifted see if the wheels spin freely with no resistance and smoothly. Sometimes it will humm while spinning by hand. Then for sure it's the bearings.

 

pull the rear driveshaft and check for a tight u-joints...rule out the easy stuff first

 

Have pulled off driveshafts individually to isolate them. They are fine. I have now isolated the issue to the rear differential of the vehicle. I will inspect this and see what bearing trouble we have there. If you have any words of wisdom let me know