I have the same issue you are having. 6.2/8 speed and 15k miles. My problem was not exhibited until I was towing a 5k boat ~1200 miles. When I got home and did some driving around town, I noticed the shuddering issue. It was exactly as described in 5337. This is a well documented issue with GM and as such, they have 5 different revisions for this TSB. The issue I had was finding a dealer that was willing to admit that there was indeed an issue and that this was not something I was dreaming up. Several told me "no MIL, no problems". Finally, after dealing with several people that thought I had a made up problem, I contacted the last dealer. I told them the problem I was having and they insisted that they did not need my diagnosis and that their Techs would diagnose it for me. Following that, I told them my diagnosis was that for a $50K truck with 15K miles on it, it sure had a weird vibration and I didn't care much for it. I had run a log to see what the TCC slip was and it was most certainly out of spec. I finally dropped the truck off and a week later, much to my surprise, the torque converter was deemed "defective".
This topic struck me because I also have a tune but had 12K uneventful miles with it. I only had an issue after towing a relatively small boat for an extended distance (should not have been an issue). If I were you, I would look in your area and try to find a dealer that isn't going to run you around and offer to fix the issue. While it may take a lot of phone calls, it may save some time in the end.
If I have anymore information once mine is repaired, Ill be sure to let you know.
Also, I noticed you said that your truck didn't exhibit this issue until after a 2 hour drive. Mine would not do it until the trans was at or above 165. I work close to home so most times I don't notice. Its only after driving 30-45 minutes or more that it starts. I didn't know if that information would help.
Good luck with it.