Noise in rear axle/drivetrain in '95 Dakota

V8 318, 5 speed manual, has heavy duty rear end, not the typical smaller one. Limited slip/sure-grip too.
I can't figure this one out. The truck makes noise, like a howling
sound I suppose, and it sounds like it's coming from the rear end to me. Seems to do it only under load, and only when I'm pressing on the gas to make the drive line work. If I step off the gas, it goes silent. Press down again on gas, you hear the howl. Usually best heard at moderate speeds, like 35mph to 45mph, although you can hear it at faster speeds too. I've put the truck up on stands and let it go, can't hear a thing that way. Nothing loose or obvious tell-tale signs of bad bearings when I turn the rear wheels by hand. Universal joints are tight.
Also, I hear a quick and short whine, kind of like gears not meshing too well together? That's just a guess as to what it sounds like. It makes that noise just as you start from a stop, for only a second then quickly goes away. Not everytime either. It usually does it when starting up an incline, but doesn't have to be. Just notice it a lot more starting up a hill or even slight slope. More load to overcome I suppose. Can't tell if this is in the transmission or again the rear end. I've heard people describe this sound before with limited slip differentials, usually when that friction modifier is not used. (Mine is using it, I added it to the gear oil several years ago when I changed a rear axle seal.)
Thx in advance for any help!
-Troy
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Sounds like your rear differential is about to go out. Pull the driveshaft off of the rear yoke, and see how much play you have in the yoke/pinion (both rotational as well as front-to-back)... I'm betting quite a lot. The howl you're hearing is the pinion and ring gears not in alignment. I'm guessing worn pinion bearings, which is allowing the pinion to deflect while under load, causing the mis-alignment.
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