Max payload is based on a stripped truck without any accessories or passengers. Step up to a higher model, payload subtracts. Add step bars, spray in bed liner, etc, subtract that weight from the payload listed on the drivers door jamb.
The weight over the rear axle, is that an "empty truck": just drove off the assembly line, or one that has accessories/ "stuff" in the bed? If "just off the line", then no, do not subtract that weight from the payload. BUT if that is your hitch and pin weight then you need to subtract that from the payload.
Actual payload = gvwr - trucks actual weight
But you still have rear axle (and front axle) weight rating as well as gcwr (gross combined weight rating=truck + trailer)
Hope this helps