I am not sure about the heavy rear bar, but it has heavier rear springs than a typical 1-ton. When it was a commercial truck, we regularly carried 3,000 lbs with it regularly and towed a 15,000 lb trailer with 3-4000 lb tongue weight 20 miles on a direct ball no weight distribution setup. I don't know if I'd do that now, but other than the hitch now having some slack in it, no harm was done.
I am not sure why these truck weren't rated higher. Compared to today's trucks they were down on power, so maybe that's it. It was also before the towing wars and seeing who can climb the fastest (great way to break something, in my opinion). Back then a 1-ton Dually was just a work truck and we really didn't worry about gvw and tow ratings. If it needed hauled, we hauled it.
Sounds like I better be pricing some airbags