Check bearings like stated... Or try the 12 and 6 oclock play on them...

 

I know I am bringing an old thread back to life, but you are describing the same issue I am having.
Very quiet but steady hum sound at speeds below 68. Once I hit 68 it gets much louder - to the point that it sounds like I'm flying a small plane.

Replaced:
Wheel bearings - front and reard
Ring and pinion and all bearings
U-joints -Front and rear
Wheels and tires
Serviced transmission
Service transfer case
Had built and balanced three... yes, three new drive lines
Checked drive shaft angle and reduced (no change)
The only thing I have not done yet is to replace the axle itself. I'm wondering if I may have a slight bend in it.

If you found a solution, I'd love it if you would share.

 

Does your humming sound change with throttle position? In other words, is it loud when you're on the gas and quiet when you're just coasting at the same speed?

 

Found out it was from the 6” lift kit causing a large pinion angle On the front drive shaft to the axle , I fixed it by cutting the outter ends of the axle housing and rotating them a bit allowing less of an Extreem pinion angle . Lost a little wheel caster but helped out a ton