Jury is still out and how similar or different the NA Ranger is from the global one. The frame material might be a little different (though I've yet to see a detailed comparison) and the NA version gets steel front and rear bumpers from the factory. By all accounts so far, the two chassis' are very similar, if not mostly the same.
Different engine options may still be in the works: Ford does have a NA-compliant 3.2 inline 5 diesel that they could fit into the Ranger and there are rumors of an overseas 2.2l inline 4 diesel being made emissions compliant.
As for different payload ratings, I don't think that's an equipment issue. To my understanding, the NA Ranger uses the same chassis components (drive shafts axles, brakes) as the global one. I've already mentioned the possible differences in frame construction, but with high-strength steel and fully boxed construction, the NA Ranger's frame doesn't strike me as the limiting factor in the payload equation. I honestly believe that OEM's rate their vehicles differently for overseas markets. An Australian-spec'd Ranger might be rated for 1 ton of payload, but I truly wonder how many Canadian and American drivers would find that it handled said weight in a safe and effective manner while driving on NA highways.