I had a 2005 Yukon XL 1500 2WD when we first purchased our 28ft 5,500lb (dry) 6,700lb (loaded) TT and and it was not a fun towing experience on any grade. Seemed to do fine on flat ground but any hill and we were down to 25-35MPH with the trans trying to drop into first. Had to replace rear axles during 2nd year. Had a TruCool 40K on it and temps stayed in acceptable range.
Looked for a 2500 to replace it but eventually settled on a 2012 Denali XL with tow package a couple years ago. I put a TruCool 40K on it and no other mods and it has been a very capable TV, although the rear axle is still the weak point IMO and my family is still growing (3 kids plus a dog). The 6 spd transmission helps and the 6.2L (great motor, minus AFM) can generally hold speed up most grades. It doubles as my wife's DD and she loves it. If we had to continue to use it to tow I think it would be fine but I'd expect the rear axle, autoride shocks or trans to wear prematurely, plus the gear ratio is still only 3.42 and it's AWD. The Integrated Trailer Brake and auto leveling is nice. But I kept looking for a 2500 I could use as our main tow rig.
Finally in January I found a low mileage 2005 GMC Yukon XL 2500 4WD with the 6.0L and 4.10 rear axle that wasn't exorbitantly priced so I pulled the trigger on it, had some significant maintenance completed and we've made one trip of a couple hundred miles and it did FANTASTIC. Maintains speed up grades with no problem, feels steadier and more solid all the way around. SO far I'm running it stock, did upsize (slightly) tires to 265/75/16 (from 245) and added Timbren jounce stops in the rear.
