This is a classic low pressure decription of wear. I would increase the tag pressure to a minimum of 100, and increase the drives and steer accordingly. I'm curious, what brand of tire do you own? Michelin, Firestone and BF Goodrich are notorious for heavy shoulder wear when under inflated. Bandag has receiently developed a 'scuff resistant' retread compound that we have had really good success with. Perhaps when your wear reaches replacement level, you might consider having your tags recapped.