You know what - I completely discounted the tires. When I heard the similar noise @60K, it remained after I replaced my tires and only went away after the dealer replaced the wheel hub. I incorrectly went in on this issue thinking it had to be the wheel hub/power train - and not even considering the tires.
So it seems I've got a smoking gun here: tires are Yoko ADVANS - 9/4mm F/R tread left noted on last oil change a couple of weeks ago. But they're more then three years worn, and due for replacement. Snapped some pics - I put one below (PBucket ain't letting me link more than one) - it's obvious wear measurements were only taken on the outside (left side of pic):
Not easily seen in the pics, but as expected, the surface of the tire curves noticeably inward towards the right edge of the pic. Would this be a good suspect on the cause of the noise?
I'll put the Z back together and start shopping; not only for the diff bushing stuff, but also a new set of shoes.