Good information. I have 10 ply Goodyears on mine, I like them alot. The tire pressure you should run is clearly listed on a sticker in the door jamb on the drivers side of everything Ford makes. It is a guide line in my humble opinion, if you keep 1600 lbs. in the bed all the time, then more in the back tires might be a good idea, if you buy a tire that is wide for the rim width, then slightly less might be appropriate for correct wear. But if you keep them filled up to what the side of the tire says all the time, then you have a giant "L" invisibly tatooed on your forehead, it will ride like crap.
And if you want to know anything about tires I certainly would take anything that a salesman at the tire store tells you with a grain of salt, putting tires on for a living part time for $8.00 an hour does not make him an authority on the subject. Check out the manufacturers website or call them for technical questions.
My 10 ply's are, I'm sure stiffer than the factory tires, but they are also 2 inches taller, so I assume the sidewall flexes more than before and it all feels the same to me. they did take more weight to balance though, I wouldn't be surprised that 10 ply's would more often than not need more weight to balance than 4 ply's even in the same size, becuase thay are heavier and have more stuff sanwiched in them.
I am not a tire expert, I am an expert at finding the truth