I have Nitto Ridge Grapplers in 35/12.50-18. I had 37 PSI in them and they rode good. I took them down to 35 to try them out. They seem to ride just a little softer and the tread wear using the chalk mark method noted below seems to be right on.
I read an article several years ago in 4 Wheel and Off Road magazine. They recommend using a piece of chalk, and drawing a line across the entire tread pattern of the tire. Drive on pavement for a short distance. The chalk should be evenly worn off of the tire the entire tread width. If the middle chalk mark is worn more, you have too much pressure and the tires will ride rough and prematurely wear the center of the tire. If the outsides wear more, the tire is under-inflated. This system works especially well when wheels and tires other than stock sizes and weight ratings are used.
I have used this method and it works very well. Most of the time, your truck will need less PSI in the rear tires due to less weight. For example, I ran 37/12.50-17's on my Cummins Ram 2500. I used 50 PSI in the front and about 40 in the rear tires. It rode well and the tires wore evenly. If I hauled a load or trailer, I adjusted the rears appropriately.