I've aired down my 19.5's to 50 PSI to go across soft sand. I made it but struggled where the other guys dropped to 20 and were able to float much better over the top of the sand. I knew better, but my buddies promised to drag me all the way if I got stuck - It would not be somewhere I would venture alone.
The truck does just fine in snow and mud. I did have to drop it into 4wd to retrieve my trailer after sinking 4" in rain saturated ground. The ground swelled with the water it collected and the tires did a good job of rutting it out even with no tire spin.