Well, maybe not a tune, but it's humming. It starts being noticeable around 20 MPH, does NOT change with engine speed, only with road speed, and goes away when turning even slightly to the right. Both diffs have Mobil 1 75W-90 in them. Truck does not pull in either direction and the front wheels seem solid when lifted, so I don't think the bearings are bad. Last thing left to check would be the rear brakes. They were all new last Dec., before I got the truck. I tried pulling a front caliper a couple weeks ago and couldn't get it off, seemed like a metal clip was holding it someplace. I didn't have that problem with a buddy's 2001 S-10. Both have dual-piston calipers up front.
Truck in question is a 2000 S-10 ZR2. My tire pressures are a little low (under 30 all around) but still equal, so that's the first order of business, bumping them back to 30 rear, 32 front.