I wrote that last September, after getting the new, smaller street tires and getting rid of almost all of my tire noise. After another ~5,000 miles the noise was getting worse, so I finally got around to getting to the bottom of it. Turns out it is a bad wheel bearing.
There's a lot of freeplay in the passenger's side bearing. The driver's side feels tight (it spins freely, just no freeplay). So I'm debating only replacing the bad one as getting the old bearing off looks like it won't be any fun. I did pull both axle shafts, so I'm that far on both sides. (And this is a Ford 9" axle, for anyone keeping score)
A lot of gear lube ran out of both sides, so I know the bearings are getting lube. And the lube was surprisingly clean. Not fresh and yellowish, but not pure black either, and no sparkles. I don't have a drain plug, so I'm thinking I won't pull the third member to drain the rest of the lube, but just top it off with fresh stuff.
I know I'm talking about taking shortcuts here with only replacing 1 bearing and not changing the lube. Does anyone want to try to talk me out of that?
^ What Gary said!