Thanks to everyone for their inputs here.
I did some looking around under the truck and I discovered a few things. First of all the U-joints appear to be fine. No play. No grease coming out.
I jacked up the rear end and set it on stands. I chocked up the front wheels and put the truck in drive to rotate the wheels. I got a better idea of where the noise is coming from.
It sounds like it's in the right rear wheel area. I pulled the wheels and brake drums. Here's what I discovered:
1. The brake backing plate is bent slightly, and I don't mean the thin metal outer part, but the thick backing plate itself. (Hard to see in the picture)
2. Also, I noticed that the axle hub where the lug nuts are mounted, moves in and out about 1/8" to 1/4". I can grab it and move it outward and push it back in. The hub on the other side has no play at all.
What could this be a sign of? Could this be the source of these awful noises?