There simply is no better upgrade for improving the off-road performance of your 4x4 than locking differentials. It's that simple. If your 4x4 doesn't have lockers, then it is nothing more than a 2x2, and here's why. Locking differentials make sure all four tires get power to the ground, which in turn keeps you moving forward. If your truck has open differentials then the power and torque is applied to the wheels with the least amount of traction, and if those wheels have no traction at all then you are going no where. In fact, most 4x4s do not come with any sort of traction aid at all, and when newbie 4x4 owners hit their first mud hole and find just two tires spinning and all forward progress stopped, they quickly learn that what Joe Salesman called 4x4 isn't necessarily four-wheel drive.
So what exactly is a locker? Within your axle there are four choices: an open differential, a limited slip, a spool, and a locker or a locking differential. You want a locking differential. An open diff takes the rotational power coming down your driveshaft and directs it to the tires, and it allows the tires to turn at different speeds around corners, which is fine on the street. But if one tire is lifted off the ground or gets into mud or sand where it loses traction, it will get all the power and will spin while the tire on solid ground will sit there like a dead doughnut. A limited slip is better than an open diff, because it tries to give each tire power by using clutches and springs. Unfortunately over time clutches wear out, and if the engine power and weight of the tires and wheels overwhelm the clamping force of the clutches and springs, then you are right back in open-diff territory. Finally there are spools and lockers. Spools are great because they are cheap, strong, and basically lock both tires together, never letting either spin unless both lose traction. Regrettably, when used in front axles they are hard to steer, and if used on the street your tires will chirp and squeal around every corner, reducing their tread life very quickly. Finally we have lockers. These differential marvels keep both wheels at either end of your axle turning, and most also allow some differentiation of speed during cornering to preserve tires from street abuse. Within the locker market there are selectable and automatic lockers. It is really a personal preference in choosing. Some folks like that their selectable locker can be turned from "Open Diff" to "Full Spool" with the flick of a switch, and others like their automatic lockers that have no extra wires, hoses, compressors, magnets, or cables. Whatever your choice, you'll be amazed at what a locker can do for you. It can make a two-wheel-drive truck into a vehicle that can actually get somewhere off-road, and it will make your pseudo 4x4 into a real four-wheel drive.
Lockers are cool; they make average trucks into off-road heroes and even poseur trucks into real dirt-dominating machines. They aren't pretty, they can't be shown off at the drive-in, and they don't make you look tough, but they work and they make four-wheeling more fun, so they are the perfect upgrade.