The Single Best Mod You Can Make
No matter what you do to your 4x4, modifying your suspension can be the most effective way of increasing its off-road performance. In the end, all the power, lockers, low gears, big tires, or flashy paint can't compare to the benefit of a well performing suspension. Sure, without an engine or tires, you can't go anywhere, but a properly designed and built suspension is the basis of a good working 4x4.
A 4x4's suspension allows the tires to stay on the ground, which is where the traction is. Even a bald tire can grab traction if it's contacting the earth with enough pressure, while a big-lugged Swamper simply flails in the air if your suspension is too stiff. Adding a locker can help in wheels-up scenarios, but why are the tires in the air in the first place? A big engine or low gearing can be a benefit to your truck as well, but without a tire on the ground you're still stuck.
Suspension also allows for the clearance of larger tires which can allow larger obstacles to be climbed, or for floating over sand, mud, and snow. But more important than clearance is the control and movement of the suspension, to keep those big tires where they belong instead of jumping off the ground because of bumps or too much power. Add the benefit of a nice ride, and you can see why suspension is the king when it comes to a 4x4. Even on the street, a well built suspension can make the difference in handling a corner or skittering off a cliff with negative results.
Suspension can be either on the supple side or stiff, depending on whether you're running a mud bog or a rock trail. Choosing the proper setup for your particular driving style, terrain, and vehicle is paramount to performance and comfort. A good suspension allows your vehicle to traverse whatever terrain you desire, regardless of your rig's power, lockers, gears, or looks. When building your 4x4, look to suspension first, then you can add real tires to fit, change the gearing to turn the tires, and then build the engine to really make you burn rubber. You'll be looking fine regardless of paint and graphics when your rig clears the end of the bog or reaches the top of the hill.