Back in the good old days trucks were built to work, and they came with a granny low transmission and low axle gears. Ever since then true off-roaders realized that gearing is the great equalizer.
Whether running stock rubber or giant aftermarket muddies, you can increase the control and capability of your 4x4 by going to lower gearing in the transmission, transfer case, or axles. We have seen amazing things done by underpowered and overtired trucks because they have super low gearing-and the reason why is torque multiplication. Every time your engine does one complete revolution per minute (rpm) it produces a certain amount of torque, and the lower the gearing the more that torque is multiplied before being put into each single rotation of the tires.
When it comes to gearing you have multiple choices in where to apply it. Manual transmissions offer the best options in driver selectability, and many of the older truck four-speeds can be had cheap. Automatics can supply smooth application and torque multiplication with their torque converters, but autos must be kept cool and some are computer controlled, which can have added complications. Probably the best choice for gearing is found in the transfer case, where getting multiple low ranges is a great option for different terrains. Whether you are running dual low-range boxes such as a 203/205 doubler or the Toyota versions, some stand-alone underdrive box, or one of the new three- or four-speed aftermarket cases, getting options in gearing can allow you to run obstacles at different speeds and with varying amounts of control while keeping your engine within its powerband.
Finally there is your ring-and-pinion gearing. This ratio is very important as you want a gear low enough to easily get your 4x4 moving, but not one so low that your engine is screaming at highway speeds. The powerband of your engine and the tire size determines the optimum ring-and-pinion ratio for your 4x4, but the majority of street-driveable off-road vehicles will fall between 4.10 and 5.38. If your engine has a bit more low-end grunt, then you can stay on the higher side (4.10s). If your engine is a little underpowered, but can happily rev at higher rpm, then you can go lower (5.38s).
In the end gearing isn't flashy, it's not something you can polish, and it won't make your truck stand out in the parking lot, but when you hit the trail you'll be crawling all over the place.