Which interior PKG did you get? I can see and have heard of 1000 lbs difference between a stripped no added options, vs loaded with ALL options in a crew cab. Their were three of us with 2005 crew cab duallies on here at one time, lightest red a few option in the 71XX relm, my LS 4WD 7400 and iirc a bit over 7600 for a full leather PKG LT version.
If you WB is 6-8" longer than a typical box delete rig, that adds a few lbs for extra framing. Same if over hangs is longer....
68 gal tank vs 35-40 gal tank......all of those add up to less payload, everything else being equal per say.
Marty