A seldom discussed aspect of GVW and therefore payload capacity calculations is the vehicle final drive gearing and cooling capacity.
Light duty and passenger vehicles are geared to deliver good EPA mileage numbers at 75mph on the I roads. 20 inch rims, axle gears for the salt flats, 2 and 3 overdrive ratios, fluffy shift management, quiet fans etc.
Just because the rear axle won't break in half doesn't mean the vehicle will be comfortable towing a barn door up a grade at 100F in a cross wind...
Do "1/2 tons" come with 4.10 or 4.56 rear axle ratios?