Time to select NEW tires

I did as Grit mentioned, ran 40-45 lbs on the rear of my now sold 05 GM dually when I was not carrying a load. Even when I carried loads, I kept them at 45 many times. Most of my hauls where short distances, so heat buildup on a freeway for many hours was not an issue. I got 90K miles before selling that truck with M55 Toyo all steel belted tires. 
I've run some sprinter and Transit vans with the Firestone HT versions. They were not the all-around best tire, but we usually got 60-70K miles out of them. We could probably have gotten closer to 100K with an equal higher quality tire, but 50% more cost. 
If they have worked for you in the past, they will continue to work for you now! Just lower the psi in the rear accordingly. Fronts I usually keep in the 60-70 range empty or loaded with diesels be them single or dual wheels. My navistar on the other hand, runs 110 ALL the time! Thats a different animal! 

Marty