This topic is state specific. In California, if you are not over on any axle or tire rating, you are good to go as far as weight as long as you are not over 80k on a big rig or doing something obviously dangerous with a pickup. The OEM GCWR seems to not be a concern in our state, nor is it listed on the door tag. Of course you need to have the proper DMV tags if you are bigger than what is defined as a pickup . Anything over 11,400 GVWR in California is not a pickup by definition, regardless of a "PK" on the title and a factory bed, etc. And in California, if your trailer has a rated GVWR of 10,000 lbs or more, you need a CDL, even if you haul it around empty with a half ton truck.
These things I have just stated will clearly be in conflict with other states, and that is my purpose for listing them. I have found the best resource for this sort of stuff is our local commercial highway patrol guys and gals at the truck scales. They live and breath this stuff and seem appreciative when people ask before getting pulled over.
I can haul nearly 30k lbs legally with my pickup and tandem dually gooseneck if I have it balanced out to maximize every axle and tire. Having said that, I much prefer the medium duty. Just look at the frame, the springs, the brakes, the unladen weight. Just because we are legal hauling huge weight with our pickups does not mean it is prudent.