There are different load ratings for tires and that is really what is more important than the number of plys a tire has. For example, a load range C tire indicates that it's a 6ply tire, but it may only have 2 thick plys.
In this case you're talking about a load range E tire, or 10 ply. This is used on 2500s or greater vehicles or commercial vehicles. 1500s can use standard load range D or 8 ply tires without a problem. Since most 1500s will never see enough weight to need a load range E tire, most people won't put them on as they are more expensive and the standard D range will be just fine for most truck owners. The standard load range on tires for 1500s is 104-114 (2000-2500lbs aprox) This will handle most all loads that truck will ever see and still provide a softer ride.
Putting load range E tires on a 1500 is just fine, they are load range 122/124 or greater and rated for about 3500lbs per tire, this is great for 2500s or greater where you might have 3000lbs of equipment in the bed. I run load range E tires because I do push the limits of what my truck can haul, and because I don't want to risk a blowout going down the highway at 70 with a 7500lb travel trailer behind me, I opt for the heavier duty tire.
As far as tire pressure goes. Commercial vehicles rarely adjust tire pressure, and run them at 3/4ths max pressure at all times. This results in excess tire wear when unloaded, but optimum control and handling when loaded. As a result, I see commercial vehicles come in all the time with the center of the tires bald while still having 5/32nds worth of tread on the outside. I also see plenty of unknowing men in their 20s with big tires like the ones you're looking at with the centers all worn away because they run them at or near sidewall pressure.
The proper way to maintain the life of your tires should you get these tires, is to adjust the tire pressure to your load. The rule of thumb is 1psi for every 45lbs of weight per tire. If your truck weighs 5500lbs, thats 1375lbs per tire, so you should fill to about 30psi. If you put 1500lbs of weight in back, you should increase tire pressure to 47psi. This will help prevent unnecessary wear on the tires and maximize your investment. For comparison, the pressure per pound on a load range D tire is about 40lbs per 1 psi.
Overall, I would never put anything but a load range E on my truck, aside from the slight expense, it rides much firmer on the road and hold up to poor road conditions a bit better.
