mlitt,
The GM trucks can be a little confusing. You could have 3 trucks with three very different rear axles, all bearing the "2500" designation in 2003. I believe 2003 was the last year GM had the option of the regular 2500.
The regular 2500 has the same drivetrain as all of the 2500s did dating back to 1988 (disregarding the minor changes in the chassis). They have a 8600 lb GVWR, 8.825 " ring gear front axle and 14 bolt semi floating rear (9.25" ring gear I think). Spring ratings were similar.
The 2500HD's with 6.0L gas engines have a GVWR of 9200 lbs, have a larger front axle (don't know the size) and have the bullet proof 14 bolt full floating rear axle (10.5 " ring gear). An interesting note is that the newer 14 bolt FF's do not have the pinion gear snout bearing like the old axles had. Gears are not interchangeable, even though they look identical. Spring rates front and back vary by package, but are generally stiffer than the regular 2500.
The 2500HD with the Duramax diesel or 8.1L gas engines are the same as the 2500HDs with the 6.0L gas engines, except the transmission options (which have been beaten to death in many other threads and other forums) and the rear axle. The rear axle for these was designed and manufactures by AAM, is a full floating type with a 11.5" ring gear.
Add to that, the various suspension packages, like the towing package, snow plow prep, etc and you have a lot of variables in the exact towing and payload numbers. When you have trucks at the opposite ends of the "2500" spectrum, its not unreasonable to see the huge difference in payload and towing numbers.
