hmmmmmmm, sorry for the misfortune! At this point, I would hate to ask this but are you sure they swapped the differential out? I don't remember what you mentioned about the dealer but are they reputable? Is the noise resonating from the exhaust? Is the noise for sure coming from the rear of the truck?
If its a brand new diff, then you can eliminate that and good score as you are now on a zero mile diff! At this point, I would be pulling wheel bearings out and checking axles and driveshafts. I'd hate to throw more parts at it but honestly, we are all sitting miles away troubleshooting without any idea of where to start with and what the mechanics did or did not do.
When my rear wheel bearing went (almost 300 miles after I bought it) it sounded like nails on a chalkboard, sand-paper on sand-paper, and all those other cool noises we hate. Jacking and moving the wheel did not give me 100% obvious indications but when I took it out, it sounded like a baby rattle. Mine only made noise when it was warm and when it wanted to.
Maybe take a video and post it.......about all we can help with from here!