What size was stock (245/75R16 IIRC?). What have a ya gone to? 265, 285, 315, somewhere in that kind of range? If it's mildly bigger size than stock, I might try filling them to stock spec (as a starting point), then run a chalk line across the tread, roll the truck forward some and see if the chalk comes off evenly across the tread surface. If the tire is underinflated, the edges will wear off first, if it's overinflated, the center will wear off first. This would probably be a good way to check, unless somebody with the same truck and tire size can chime in and give their specs!