Tom,
You are correct in the.general thinking of do not go over an axle weight per lowest denominator, be it rims, tires, springs, axle tube itself. Those numbers from a pickup standpoint, will be less than max legal limits as noted.
Also, what is paid for can trump too. My C2500 has a door sticker of 8600, axles total around 10,000, tire width measurement gives me around 9500 an axle, but paid for license is 8000 lbs. Reality is, any.time I am over 8000 lbs on. My four tires, I am over weight and illegally running down the road.
Choose poison on how to do things. Keep all of the how and what you have to follow in mind. So you are both safe, and legal!
Marty