I was close to pulling the trigger on a duramax swapped Suburban once. I ended up keeping the Yukon Denali for the wife and got an LML as a spare/travel/towing only vehicle. Still kind of dream of a duramax Suburban....
Yes, I have towed relatively light with the Yukon XL (6.2, 6L80) on a multi hour trip. It wasn't the most fun. I couldn't imagine towing 6-7,000lbs across multiple states with it. It does fine for short trips.
I put a shell on my 4 door duramax when i want to scratch that itch,lol.