In general, not for rock crawling, but for comfort during mild off-roading, how much can you safely air down a 7K LB truck that is unloaded save for 1 or 2 people on board? I'm on stock 20" rims and BFG K02s, truck is diesel SD, Crew Cab, LB, SRW.
There's not one pressure and the correct one for any truck and surface is going to depend. You don't want to go so low that you risk popping the bead or risk beating up rim lips.
What do you normally run on the street? I assume it's going to be pretty high on a truck like yours. Like 50 or more? Are you running tires with exceptionally stiff sidewalls? It's not just a question of load range. I run load range E tires and some have considerably stiffer sidewalls than others. I don't have to lower the pressure on my Falkens with 2 layer sidewalls as much as I did with my BFG and their 3 layers to achieve the same effect.
Going to 25 psi is significant I imagine in that case but might be right or even not low enough on a beach or deep snow while 35 psi is all you want to use for taking the edge off fire roads.
And you might not want to use the same psi front and rear. Without cargo the front end of your pickup is quite a bit heavier than the rear.
I'd suggest doing some tests in the driveway, let out 5 psi and see what happens. Then 5 more, 5 more, 5 more, etc. You might not need to let out much to help and you can always burp a little more if you want but airing back up if you've gone too far is more of a PITA.
Chevy 2500HD XC LB 265/75-16: For dirt/FS roads I generally drop by 50%. With my typical load that means around 30# front, 35# rear. If I hit bad washboard another 5# or so until my teeth stop rattling around. Ex: I run 25/30 on the Mojave Road with no issues. I think most “modern” wheels and LR-E tires will hold a bead at pretty low pressures if not rock crawling.
So, you have lower profile 20" tires, like 50 or 60 profile? Better keep these at a higher pressure than a truck with 16, 17, 18" wheels and a taller 70 or 80 profile tires. The lower profile pops off much easier.
My tip is to buy some cheap take-off 18" wheels and mount a higher profile tire. Coming out of winter you can find some great deals on mounted snow tires on wheels. With these, I'd run 45/40 psi F/R on the highway, 35/30 on gravel, going down to 20 on sand/mud. Of course 60+ when loaded. I bought the AlfaOBD program to set the tire pressure nanny to lower pressure.
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.