UUUUGH The u joints checked out, shop said they're fine. They couldn't figure out where the noise was coming from and when I drove around with the mechanic, of course the noise stopped. It didn't happen at all yesterday either, and then was especially loud today until I got to the shop and it magically disappeared.
They said if it comes back bring the truck back and we'll start tearing it apart, but until then, hard to say what else it could be.
Y'all have any thoughts I haven't covered? So far the brakes, bearings, tires, ujoints all check out.