Get a photo of the load sticker (on the door jam). That has all the info. You may need to go to the manufacturers site to decode some of the info. But payload is now clearly listed. It will have details about the rear-end ratio and such specific to the truck.
If buying, I would lean toward the 1 ton SRW. For the most part the same truck but higher payload, so the 5er limits will be better
Pin weight is usually the limiting factor and for a 16k lb 5er, that can push 3500lb plus a couple hundred for the hitch plus anything else you load. So while you may be able to pull 16k lb, you might not be able to lift the pin weight that goes with it.