One thing I didn't think of before, but I wonder if you put some grease into each of the rear u-joints, one at a time and then drive it to see if the noise reduces a bit. Greasing them one at a time and then driving would isolate which u-joint it may be. My thought is if the u-joint is bad but doesn't have a lot of play yet, maybe a shot of grease will quiet it down a little letting you know it is an issue. It would be pretty quick for you to do.

Running it on the lift while someone listens from below sounds like a good idea. If there was a way you could mount a GoPro under the truck while driving and putting it in different positions, one near the u-joint near the transmission and one near the u-joint at the rear differential, you could get some footage and hear the amount of noise coming from those different camera locations. If you have a GoPro, they do make different adhesive mounts that might work.

I had a u-joint go bad on a 68 Ford Bronco way back in the day. It was making a lot of racket and I had no clue what it was. At the time, I was working at a Chevron gas station and one of the senior mechanics listened to my truck while driving and immediately knew it was a bad u-joint.

Hope you get it figured out.