Alright, wanted to provide an update on this.
This afternoon, I went out with an intent to truly tackle this once and for all. I did not attempt anything from the interior of the vehicle, but focused on the exterior. I focused on the rear left wheel well area first.
1. I removed the liner, and put some felt padding on two nubs that touch the fender.
2. Once underneath, I noticed what looks like an EVAP pipe that goes up the fuel filler area that was flopping around. I was able to make it rattle a bit while flopping it around. I secured it with some zip ties.
3. There is a hard brake line that goes to the rear left wheel, and goes across the differential area. I found that by tapping on it, it hits a crossbar and can rattle. I put some styrofoam padding underneath it as a temporary measure until I can get some rubber piping as a more permanent solution.
4. On the rear right wheel, I put some padding on the wire loop that hold the brake line, as I could tap it and make it rattle. Once again, temporary until I can get some rubber piping.
5. I secured anything that could rattle around with zip ties.
For good measure, I got on the roof, and found the rack bars were a bit loose and not straight. I straightened and tightened them up. After this, I took the truck for a long drive.
Doing all this, the rattling has been
substantially lessened!...but not eliminated. I wonder which of these would count as the culprit? Something is telling me that it might be the roof rack bars.
I will be trying this next:
http://www.tahoeyukonforum.com/threads/2015-yukon-roof-rack-cross-rails.75184/
http://www.gm-trucks.com/forums/topic/162069-rattle-in-rear/
