Checked fluid level in diffs and all good with yellowish color, no dark or anything unusual. Going to drain a bit this afternoon and see if there's any large metal debris. No leaks from the diffs either.
Going to check for pinion backlash play by lifting the rear, putting truck in neutral, and move driveshaft back/forth/up/down and if there's any movement or unusual noise.
The final test will be driving at 40-50 on a straight road and putting truck into neutral. If noise disappears instantly it's strictly the gear mesh pattern. If howling stays exactly the same is structural or bearing related.
Something I should have considered, but this is all new to me, is to use a solid sleeve instead of the collapsed crush sleeve. Apparently, when towing heavy the pinion gear wants to get pulled into the ring gear. This forces the pinion shaft forward against the outer pinion bearing. The extreme thrust from towing can compress the sleeve even further creating slack.
I took a trip last month and towed our trailer for a week with a lot of extra weight in the back of the truck. I started noticing a howling noise but thought it was the tires.