Josh,
We've had a number of discussions about whether it's legal to tow something heavier than your rated capability. While the opinions expressed on the forum aren't necessarily sound legal advice, and every state and locality could be a bit different, our general conclusion regarding non-commercial vehicles is that it's not a statutory issue. No one has posted about firsthand experience with law enforcement citing them for being overloaded. And I don't know that anyone has ever pointed to a specific statute.
We have had many opinions about whether it's safe to tow when you exceed the rated payload. I would say that the more overloaded you are vs your payload spec, the less safe you are.
We have had many opinions about whether the drive train and axle and wheel/tire components are always the same between 250/2500s and 350/3500s. My conclusion is that on some makes/models/years perhaps it is. On others, perhaps not. The suspension is probably not the same.
Notice that I'm talking about payload - weight on the rear axle - pin weight plus other stuff. The towing capacity of the engine and transmission, and often the drive train may be the same on 250/2500s and 350/3500s, regardless of how it's rated. Payload is not the same.
We have also had discussions about liability if there's an accident. Could a lawyer demonstrate that you knowingly were towing in an unsafe configuration? Could he convince a jury that your indifference caused harm to someone else? Could liability limits be lifted if he's successful? Maybe - maybe not.
But of all this, I wouldn't get too upset with the government. They seem to have little involvement.