As promised, here is the tread depth/tire condition update:
Both front tires read out with a tread depth of 4.4mm or .1795". The rears are not quite so perfectly matched, but do read a little deeper at 4.71/.1865 L, and 4.74/.1875 R. All measurements are taken in the outermost tread groove.
As you can see in the photos, the wear is pretty even, and there is still some room before the wear bars will be touching pavement.
Being the factory rubber, they have only been rotated once.
This afternoon, I put in a call to the dealer, where I had the tires rotated at the same time as an oil change. That was almost 20 000 km (12 k miles) ago. I asked about the precedure for tire rotation; crisscross pattern, or straight front-to-back, and before I had finished asking, the service guy interrupted and told me they never cross sides, because it can cause the belts to shift and ruin the tires!
So, you anticipated the answer very well, Klugster. In fact, many years ago, my dad taught me about this, saying that in the old days, with bias-ply tires, it didn't matter which way you swapped them around. They even used to advise swapping the spare into the rotation to extend the life of all five; an idea from the days of full-sized spares, naturally (I know this stuff because I'm OLD). However, said Dad, with radials, if they've been run any distance, changing sides, and reversing the rotation can actually make them fall apart.
Personally, I still have a sneaking suspicion it's bad bearings, either front or rear. Noticably warm wheel centers on the rear (but not the front) after 20 minutes of highway travel just tends to confirm this idea. I guess time, and miles, will tell the tale.......