Gottahaveit is right on. The 2500, AV 1500, AV 2500, and all 'burbs; share the same chassis. This is the chassis without the room for the Allison or the ZF 6spd. These two trannys are mandatory for the torque of the Duramax.
The 2500HD and 3500 have a little bit higher stance, body lift, frame difference, whatever, it allows for the big trannys.