Whether you have a dually or SRW the trucks are pretty much identical as far as their capability. The dually obviously can handle more pin weight. Will your Denali have 20" rims? The 20" tires on my Ford are rated for 3750 lbs. The unladen rear axle weight of that Denali is likely around 3500 lbs, so a 4000 lb pin weight would put you right at your tires' maximum load rating. So, like people are saying you are on entering dually territory. I would be OK with hooking onto an 18000 lb fifth wheel with my SRW for occasional use, but, if towing 18000 lbs was the main purpose of my truck I would get a dually. On occasion I tow up to 30,000 lbs with my SRW. This is when loading a tandem axle dually gooseneck where I can control my pin weight and obviously a dually would be a better choice for those heavy occasions.
