Harmonic noise @ 45

I have a 1992 2500 4x4 with a Getrag. I am in the last stages of rebuilding this low mile rig to be a farm work truck. This truck has a low harmonic noise that starts in @ 45. This noise is constant and does not change with either power or coast. I have changed all of the wheel bearings and rebuilt the transfer case. The differential has a nice wear pattern and tight bearings. With a remote stethoscope attached to the differential, I can not pick up the hum. The stethoscope does however pick up the noise at the transfercase both before and after the rebuild. The noise is still present with the transfercase in nuetral at a coast. I have checked all of the moving bearings and am at a loss finding the noise. The drive shaft has been checked and is good to go.
I have also tried different tires with no change. This is one tough customer to find.
Chime in with ideas, GEG

I took a closer look at the differential today. The smaller spyder gears are eaten away with giant divots. It was also full of metal on the magnet. I am going to change out the internal bearings and put in a new set of spyders. The local differential shop and junk yard both tell me that this is common. I will also take a close look at the ring and pinion when I get them out.
GEG

Rear axles are looking like the problem. The bearing races were both in need of replacement. One was laddered and the other was scored. These babies looked OK but when I listened to them under load with a remote stethoscope it was obvious that they were the cause. The local 4x4 shop took one look at the axles, and explained the any wear on this kind of axle will promote noise. I am going to rebearing and put the new axles in and give it a run. Should solve the noise.
GEG