The payload rating should be on the tire load sticker in the driver's side door. Things you have to consider with payload what you already listed and the tongue weight of the trailer.
There is a nice
2005 Nissan Towing Guide which lists available configurations and their respective towing capacity numbers (pg 25). One thing to note is that even though it has a hitch, it may not have the towing package. Without the towing package the specs for that truck look quite low and that trailer would be too much for the truck. You might try a Nissan forum to get more info on how to find the RPO codes and see what the truck is really equipped with. Until you have a better handle on how the truck is equipped its difficult to make any recommendations.