There are a few locker types out there:
Mechanical lockers aka lunchbox lockers aka unlockers: these have spring-loaded side gears that keep the diff locked most of the time, but will disengage (unlock) when making turns to allow one tire to rotate faster than the other. They'll make a click-click-click or tap-tap-tap noise during the turn. This allows them to be streetable to some degree, but loose terrain (including wet road) may prevent them from disengaging, which could prove hazardous; simple solution: slow down, brake earlier. Because they are already locked, there's nothing extra the driver needs to do to engage them. Most lockers of this type fit inside the OE differential carrier; some (like a Detroit Locker) replace the carrier. If you have rear LSD, you'll need an open carrier to put non-Detroit mechanical lockers in. These are rather economical options, running around $200-$300 for most brands, and can be installed with basic tools.
Air lockers (ARB) or Pro Locker (TJM): these require an air compressor and some electrical wiring to engage. The air pressure is what locks the locker, and the locker unit replaces the entire OEM differential carrier. The wiring portion puts electric solenoid on the air line, which opens to fill the air hose between it and the locker. This is engaged by a switch that the user toggles. One air compressor can power both front and rear locker, so it usually makes sense to run dual air lockers, budget permitting. When not engaged, the locker acts like an open diff. These lockers run around $1K and require more advance mechanical skills for installation, as gear setup is required. Factor in another $200-$400 for the air compressor and fittings.
E-lockers: while not readily available in the US, an Australian brand, Harrop, makes the Eaton equivalent of their E-lockers for Nissans. Like the air lockers, these replace the entire carrier and are open diffs when off. They don't require air, of course, but do require some light electrical wiring. A 12V source and a driver-operated switch engages the locker. Harrop and other importers sell them for around $1200-$1300 USD depending on the exchange rate. Although they are more expensive, they also don't need an air compressor.
Cable lockers: Although I don't think there's a Nissan option, cable-actuated lockers (like OX) do exist. Same concept as the air and e-lockers, except they are engaged by lever and cable. However, there are kits that can convert them to air-driven.
Spools: these replace the carrier internals, or the entire carrier. Not sure if aftermarket options exists for Nissan, but these "lock" the diff 100% because they completely eliminate the "differential" function of the diff. The idea is that all the moving pieces are held stationary to each other, which means that you can weld all the internals together to turn the diff into a spool. Cost in that scenario is just labor and welding wire.
Since you will be installing manual hubs, these tend to pair well with mechanical lockers as a budget-friendly option. Since you have a WD21, you also have rear mechanical diff options if budget is an issue. Either route, lockers are worth every penny.