Mike,

With as many cars as you've got and the depth of repair you obviously do, I am sure you have looked at everything I might suggest. But that's never stopped me before. ;-)

With a noise that increases with road speed and worsens under load, the only things I can think of are driveline alignment, u-joints, and pinion bearing. I suppose the ring gear might be noisy but I think it would only change tone, not intensity, under load. I've never dealt with the carrier bearings so I really don't know the failure mode there. Is there excessive vibration involved? If so, I'm back to u-joints and alignment.

The only thing I can think of is to drop the driveshafts and inspect them closely. If they are OK, next would be checking the transmission tailshaft to pinion angles. While the driveshafts are down, spin the pinion yoke and listen for noise or crunchies. That's the end of my experience. Let us know what you find.
--
Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 51 year old fat man ;-) -- KD5QBL