Okay, I know it's hard with just a video but we're at our wit's end.
We've banged on pretty much everything under the truck without being able to reproduce the sound when it's in the garage.
Pull out onto the road and it's almost instant and constant. Somewhat worse when going over bumps but also present with just driving vibration.
The exhaust has been thoroughly beat with a mallet, the hubs have both been checked, we've run with the brakes engaged, both hydraulic and emergency, shimmed between the gas tank and the straps, shimmed any gaps in leaf springs. The sound is present in drive as well as 1 & 2 and even reverse, and also present without the engine running (as shown in the video).
The sound appears to be just in front of the rear wheels and seems to be roughly the same volume on both sides of the truck.
Short of strapping someone on under the truck during a drive, I just can't figure out how to find this noise.

Looking forward to the guesses.

Thanks