The way to do it is to weigh the truck (both axles) and then search the web for a load/inflation table (any brand will do - it's the same for the identical size). That will give you the minimum pressure for the load on each tire. That's your starting point and will give you the softest ride. You may want to bump the pressure up from there a bit if it feels too squishy. Matching the pressure to the load will give you the best wear also.
You could also do the chalk test. Make a chalk mark across the tread and go for a drive. If the center wears off and leaves the edges of the mark intact, you're running too high. Same for low - the edges worn and center intact.
On edit - here's
Toyo's Load inflation table Look on page 23 for your size.
