I have a tandem axle boat trailer with excessive wear on the outside shoulder of both tires on the rear axle. The shoulders are bald with less than 5000 miles on the trailer. Both tires on the front axle are fine. I check the tire pressure before any lengthy trip and the boat is within the trailer's weight rating. The trailer generally seems to ride pretty level. Combined weight of the boat and trailer is around 6000-6500 pounds. Any thoughts as to what to check?