The payload is limited due to the 10k GVWR limit of the class that the 2500 is in, and there for the payload is also capped. The rear suspension and rest of the truck can handle considerably more than the rating. Only 500 lbs separate the rear GAWR's of the 3500 and 2500 and the rest of the truck is identical, however, the 3500 is not capped at 10k GVWR like the 2500 is.
Not saying the 2500 can handle as much as the 3500, but what it can safely and reliably handle is pretty darn close from my experiences. I have had over 3k back there and it handled like any 3500 and only squated enough to make the truck level.