Thanks for the replies everyone. My first inklings that something was wrong was when a tire guy looked at my tires and said that I was rolling the tires onto the sidewalls when turning. After reading some of the comments posted on this thread, I dropped by a different tire shop this morning and asked them to have a look. According to them, the marks on my tires are not caused by rolling over onto the sidewalls but caused by curbing. That confuses me because I don't remember parking or driving near curbs. My driveway is wide with no curbing and the places I go to all have docks with flat level pavement everywhere.
I originally started out with the mfgr's recommended tire pressures of 50 psi front and 72 psi in the rear, which was fine with the truck empty. Once I put the weight on, the truck felt squirrely when driving so I raised the pressures to 80 psi for all four wheels. It now drives and feels solid though I'll have to do the chalk line trick once I figure out what's going on.
On my way home tonight, I dropped by the scales and weighed my truck. Total weight was 8900 lbs with 5120 on the rear and 3780 on the front.
I guess what I have to do now is to clean off all the scrub marks and watch to see when they appear.