The advice given above for standard profile truck tires is not quite applicable to your case, but the general idea still works. Unfortunately, 20" wheels on tires less than ~35" tall would be considered low profile in most circles. While airing down a low profile tire won't provide the same gains as airing down a standard profile tire, it does still help, and I would encourage you to do so. This applies not only to off highway travel, but also to empty truck tire pressures in general. Your empty truck does not need max pressure in the tires, and both tire wear and ride quality suffer as a result of too much pressure for the load scenario, particularly with the power today's trucks are capable of putting to the rear tires. Zuber's advice is all sound, including changing the TPMS nanny to let you run lower pressures when you are empty.
How low should you go? It always depends on the circumstances... Because you have less sidewall to "flex" with a low profile tire, you are limited as to how much ride improvement you can achieve with lowered pressure without risking rim or tire damage... On the plus side, I should think even just droping to 45psi in the front and 35 or so in the rear would provide a noticable improvement on rough surfaces without inducing too much risk. If speeds are kept low, lower pressures should also be possible. Going too low will make the sidewalls heat up quickly, and will put the rim much closer to rocks and things that can damage wheels and sidewalls. One way to know if you are too low is to feel the sidewall. If it feels more than a little warm at lower speeds, you are probably at a lower pressure than you should be.
Don't be afraid to experiment some until you are comfortable.
