I was looking that up awhile ago, not that I remember it all.
Until you stepped up to the C3500HD with was 15000pound gross, the next biggest stuff at 12000pounds gvwr (cab and chassis) with the highest towing capacity would have been a 2wd reg cab with a big block or 6.5 diesel.
The standard issue stuff came heaviest at 8600 or 9200lbs in the 2500 or 3500 trucks.
The 3500hd had a doubled frame for the center most of the truck.
Cab and chassis 4x4 stuff came with a BW4470 type transfer case.
Not sure what else was different, I think I got all that right. Or very near, I would imagine model and year and options changed a bit thru the years
Edit (regardless of gvwr, the highest towing capacity was 11000 pounds I believe) or something something