Thanks all - the sound is almost gone - here's an update..

I drove the car home from the storage garage, listening the whole time. The whine came only "off throttle", and a lot of the time just not there at all. The sound wasn't really *that* loud after all.. was I dreaming the time I heard it so loud? ...maybe it's quieter with stiffer cold oil?

The next morning, I jacked the rear up, got the driveshaft off the yoke and checked how much play there was in the pinion bearing, just using my hands to yank on the yoke. There was no sideways play, and just a tiny bit of endplay... 1/16"? I noticed a drop of oil hanging from the bottom of the chunk, although nothing had dripped on the floor. The oil looked like it was probably weeping from the pinion seal. I checked elsewhere on the chunk and decided to check the torque on all the axle housing nuts. Well, I was able to tighten each nut a good full rotation. Even though I don't know the torque spec, I tightened them all as equally as I could. I did check the diff oil level last fall, it was right there near the edge of the filler port.

The whine is now imperceptible at speeds above 35-40.. above that speed the noise of the car itself makes it impossible to hear. I'll keep an eye out for oil drips and the oil level.
So thanks for walking me through this..
- Art