my .02
I can't speak for other years but on the 99's the frame, springs, etc are the same for the 2500 and 3500. What is different is the outer bearings and brake drum on the 3500 to allow for the dually rim offset. The rear drum and shoes are 1" wider and wheel cylinders are larger too.
The 3500 Cab and chassis is a different beast than the 2500 or the std 3500 with heavier frame and springs.
From what I can tell... at the factory a 3500 gets different brake drums and bearings in the dana 80, bolt on spacers on the front end, rear fenders and of course 6 dually rims. Other than obviously four tires can carry more than two the dually IS NOT inherantly (sp)stronger than the 2500.
I get sick of people with 3500 trucks acting like the 2500 is going to break the frame or something if you put a load on it. It's the same truck with training wheels.
Door decal load capacity is a different story.