A lunchbox locker is a drop-in, light-duty type of automatic locker made for most Jeep, truck, and SUV differentials. Lunchbox locker is actually a slang term used to describe this type of locker because, much like a lunchbox, the outside packaging stays the same whilst the contents inside of it are changed. Metaphorically, the differential itself is the lunchbox and the locker is the contents inside of it.
Lunchbox lockers operate just like their full-size counterparts. They’re designed to automatically engage and lock both wheels on an axle at the very instant it detects one wheel receiving more torque than the other one. This locking action applies 100% of the driving force to both wheels, essentially creating a spool. When the torque difference between each wheel is reduced, then the locker can unlock and allow differentiation of the wheels which is beneficial for maneuvering around corners.
The lunchbox locker has its pros and cons.
The main advantages for these drop-in lockers is backpack and lunchbox set that they are simple units which are designed to install into a factory open differential carrier (and sometimes even in factory limited slip carriers). In most cases, the carrier does not need to be removed from the axle which means the ring and pinion gears do not need to be reset. In the few instances where it’s necessary to remove the carrier, it’s still not necessary to reset the ring and pinion because the pinion is bolted to the carrier and as long as the carrier gets reinstalled with the correct shims that were already in place, the ring and pinion backlash will remain the same. This is very advantageous because the expertise and tools necessary for setting up a ring and pinion from scratch can get expensive quickly. If the time comes that you no longer want your lunchbox locker, simply remove it and reinstall the stock differential parts and your differential will act just as it did when it was new.
The main disadvantage is that they are only as strong as the carrier upon which they are installed in. For vehicles with smaller axles such as the Dana 30 and Dana 35 which are popular among the Jeep Wrangler and Cherokee lineup, this causes some limitations, primarily with tire size. Anything over 33-inches is not recommended for use with these small units, however there are plenty of off-roaders who will testify that a lunchbox locker was able to turn 37-inch tires without any incidents. It largely depends on the type of driving that is done. Obviously, the more extreme the terrain and the larger the tires installed, the more likely it will be to experience a failure.
In sum, a lunchbox locker is an ideal candidate for anyone that is getting in to recreational off-roading. The traction that they offer for their minimal price (most between $200 to $300) is a huge performance gain. A 2WD vehicle with a locker can perform almost as well as a 4WD vehicle with open differentials. Couple that with the fact that they can be installed by the home mechanic with basic garage tools and you’ve got yourself a recipe for some serious off-road fun.