lunchbox locker = simple (garage install, DIY).
You do not need to adjust things like backlash, and other gearing (ring & pinion) meshings.
The lunchbox lockers simply drop inside the carrier, replacing the spider gears.
ARB = shop install, unles you are adventurous enough to tackle the meshing, backlash, etc.
Those types of lockers replace the carrier. That is the difference in a nutshell.