Since I was paged...
I've done a ton of research on the 3500HD Suburban. If the right one comes along, I may very well buy it.
Here's my current rig...8600-lb trailer, fully loaded with family and gear I'm just over my 16,000 GCWR, right at my max GVWR (8600), and right at my max RAWR (5500). No WD, no sway control, rig is dead-nuts stable, even driving on interstates in Wyoming. We've dragged it over Loveland Pass (11,990'), Powder River Pass (9,666'), and through the Eisenhower Tunnel (11,100', aka the Ike Gauntlet).
Weigh slips - with trailer and without trailer:
Another comment was made about towing at 70% or 80% of your max rating as a "safety margin." Well, ok, but if you
really know your equipment, you know which rules can be bent or broken. For example, my RAWR is 5,500 lbs. The RAWR of the 3500HD is 6,200 lbs. They both have "E" tires, and they both have the same AAM 1050 14-bolt rear. Why the difference? Some GM pickups are rated at 6,000 lbs with the same setup. All I know is, my tires have a load-carrying limit of 3,042 lbs., or 6,084 total. AAM rates the axle itself to 8,600 lbs. So is it really dangerous to load my rear axle all the way to 5500 lbs? Do I have a "safety margin?"
As was mentioned previously, the 3500HD was such a low-volume production run for GM, they didn't bother paying for the towing certification test, so the tow rating reverts to the default rating for this vehicle class, which is 3,000 lbs.
Doing the math based on the door stickers of the one vehicle I inspected/test drove, subtracting the payload rating (4,267) from the GVWR of 11,000 lbs yields a curb weight of 6,723. Do that for my 2500: 8600-2088=6512. So the 3500HD is only 200 lbs heavier than my 2500. That would lead me to believe the frames are very similar, if not identical.
The 3500HD basically has the same running gear as the older GMT900 2500 trucks with one big exception - it has 4.10 gears instead of 3.73. And, of course, bigger/better brakes. Now, if you look at the tow ratings of the GMT800 2500s, the last trucks to have two axle ratio options, the big-blocked SUV with 3.73 is rated to 10,000 lbs, and the 4.10 is rated to 12,000 lbs. So the 4.10s add 2,000 lbs of tow rating. Now add the bigger brakes, and you have a vehicle that I would feel comfortable pulling 13,000 to 15,000 lbs.
If I bought a 3500HD, and I planned to tow that much, I would certainly upgrade the receiver to a class V to create something completely bomb-proof. I'd also probably switch at least the rear suspension to airbags. I'm very spoiled by the half-ton ride of my 2500. I'm currently driving my company's F250 work truck while my company car is in the shop. I have no desire to live with such a horrible ride in what is primarily a family vacation hauler.