I’ve seen rumbles like that disappear after I replaced one of the halfshafts but you checked the u-joints. My experience wasn’t with a Blazer, though
How old are the U-joints? Are they original?
Wheel bearings are my number one guess. If you pretend you are at Le Mans and do aggressive left and right turns while the noise is happening, and the turns effect the sound I would get the wheel bearings checked. A stethoscope can be very handy, not while driving of course.
RemcoW - I would guess the U-joints are original, but I couldn’t get them to budge at all when I shook the driveshaft.
Barkydog - The thing that doesn’t make sense to me is that the vibration stops as soon as I let off the gas. Do you think the bearings would respond to that?
Both front bearings on my Blazer went bad after I bought new tires. I thought the noise was from the new tires. I don’t recall any vibration. The noise changed when the steering wheel was turned off-center (i.e. changing lanes or turning).
I had a vibration at 60-65 mph in my wife’s 98 Ford Windstar. The lug nuts on one tire were coming loose a month or two after I changed the tire after a flat.