With my 2011 GM 2500HD the difference between it and the 3500HD was in three areas and only three areas. The 2500 had 17" rims while the 3500 came with 18" rims. The 3500 had an extra set of leaf springs. The 2500 was available with the extended cab and the standard bed but with the 3500 I would have had to choose between a regular cab and a crew cab.
I wanted the extended cab with the standard box so I bought a 2500HD and spent $445 on a set of SuperSprings. It took me all of an hour to install the Supersprings and they work perfectly with a 3700 lb. camper load in the bed.
Neither the tires provided on the 2500 or the 3500 models were adequate for the camper load so I replaced the ones on my truck with Nitto tires rated 555 lbs. more per tire with a gain in payload at the rear axle of 1110 lbs. with this upgrade. My truck has a rear weight when empty of 3200 lbs. and with the camper in the bed the total load on the tires is 6900 lbs., hence the need to replace the factory tires.
My 3700 lb. camper when in the bed of the truck puts an extra 3600 lbs. of load on the rear tires and an extra 100 lbs. on the front tires so I care only about the rear of the truck. The axle and wheel bearings are rated for 10,900 lbs. so that is not a concern.
If I wanted dual rear wheels I would have had to buy a 1-ton truck. I wanted SRW so the 2500 truck was not a problem. If I wanted still more load capacity I can always go to 19.5 rims and tires.
With all but the DRW 1-ton used trucks you are probably going to need to add to the capacity of the factory springs and upgrade the tires and possible replace the shocks.
There are also differences based on the model year and truck type. GM upgraded its truck frames in 2011 and Ram upgraded its 1-ton frames in 2013 and its 2500 truck frames in 2014. With Ford the F-450 has less of a payload than the F-350 with the pickup version. To get a greater payload with the F-450 you have to buy the chassis cab version which is quite different.
