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