I'm by no means an expert but figured I'd throw my comments in for you to think about.
1. Its tough to "teach" someone how to drive offroad but it is possible. As others have mentioned it mostly comes from practice which entails a generous amount of trial and error. Think about when you learned to drive manual transmission the first time, it wasn't smooth and it wasn't pretty but eventually you figured it out. If you haven't learned to drive manual transmission yet thats a good place to start when learning the fundamentals of how gear ratios work (low vs high, 2nd start, etc...).
2. Don't over think it. You know how the vehicle should feel and when its feeling good vs when its struggling. On something as modern as a 04' GX470 you won't hurt the engine or drivetrain by choosing the wrong gear since it will mostly figure things out for you. You'll feel when you are going through something muddy or up a steep incline that 4 low would provide the additional torque you need (even if you are in high and its bogging). You'll know this because you will practice driving off road in 4 low and get a feel for how much torque you have.
3. Seriously, don't over think it. There's no exact formula that says you need to be at 2000rpm when doing this, vs 4 low doing that, vs center diff lock doing the next thing. Start out with the same configuration you use to drive on road and experiment from there. 4 high, in drive, regular air pressure, regular suspension height. Explore with this setting and see how it feels before making adjustments. See some larger rocks in the road, adjust the clearance upward. See a large puddle with a questionable outcome, stop and get out to investigate. It looks about a foot deep but the bottom is solid so you are good to go, maybe click the center diff lock for good measure. What if its 2 feet deep and soft at the bottom? Things start to get a bit more serious and you should start thinking about what happens if you get stuck (recovery points, gear, another truck with you, etc...). Thats also when you go with 4 low, diff lock, and a bit more aggressive approach.
4. Always experiment. Go down a steep hill in each gear in 4 high, then try it in each gear in 4 low. Feel and compare the difference. You will be impressed with the engine braking in low range and will use it to your advantage. By selecting 1st gear in 4 low you are maximizing the available torque for not only uphill travel but also braking for downhill travel. Its a lot easier on the truck to idle down a steep hill slowly in low range than to grind the brakes and fight the ABS the whole way in high range. On the flip side it makes no sense to drive in 4 low on an easy dirt road going 40mph with the engine revved out. In that case kick it back into high and lope along at 1500rpm in high range.
5. Look ahead and make a visual plan in your mind. You are at the top of a hill and are about to descend into a steep section that is rocky with a small creek crossing at the bottom. This is a good time to stop and go for a short walk to explore. Getting stuck isn't fun and by knowing when to check things out you'll save yourself a lot of hassle. During your scouting walk you find a bunch of ruts and turn around marks (from someone else) before the creek because it was actually much deeper this time of year than anticipated. On the other side of the creek there are no recent tire marks. These are clues that its probably not a good place to cross and by going for a short walk you saved yourself some major headaches and can easily backtrack from your current location. You return in a month and things look better, the creek is low and you have some recovery gear - go for it.
6. Don't beat on the truck unnecessarily. These trucks are tough but there's no reason to slam into stuff or bounce it off the rev limiter.
7. Even stock these trucks are very capable. You will be surprised what these trucks can do totally stock with a capable driver. There's a saying that a good race driver is worth an extra hundred horsepower - its the same idea for off road. A stock GX470 with all terrain tires will take you places that make you uncomfortable. Explore these limits before going crazy with modifications.
8. Go with other people. You will learn a ton in a short amount of time and if things get hairy you'll have someone to help out. This is the most important point. I see a ton of knowledge sharing at every club meet, from guys like yourself who are new to offroading to the most seasoned veterans there is always something to learn.
Have fun and let us know how the next outing goes!