The frame is nearly identical to a 2500HD frame all the way back to the last ~2 feet behind the rear axle, it necks down to 1500 size by the reciever hitch. The 1500HD's don't have a 1.5" body lift because of this. All of the frame before the rear axle is WAY beefier than 1500 stuff. They definitely don't have a 1500 frame though. Front suspension/brakes are identical to a 2500HD (torsion bars may be stiffer on a 2500HD), and they have either a 14 bolt semi floater rear end or dana 60. The biggest difference between a 1500HD and 2500HD besides the semifloater is that a 6.0 1500HD could have 3.73 gears, while all 2500HD's had 4.10's.
I love my 1500HD, the 3.73's leave a little to be desired though. Braking power is light years ahead of my sierra denali with the same engine/towing capacity. Mine has hydroboost, not sure if all do.
EDIT: the 1500HD will have the NP246 auto-trac transfer case as well, while a 2500HD won't