A Lunch Box Locker, also known as a Pocket Locker, is a form of Locking differential

Locking differential

A locking differential, diff-lock or locker is a variation on the standard automotive differential. A locking differential may provide increased traction compared to a standard, or "open" differential by restricting each of the two wheels on an axle to the same rotational speed without regard to...

 that is installed inside of the differential

Differential (mechanics)

A differential is a device, usually, but not necessarily, employing gears, capable of transmitting torque and rotation through three shafts, almost always used in one of two ways: in one way, it receives one input and provides two outputs—this is found in most automobiles—and in the other way, it...

carrier. Because the differential carrier does not usually have to be removed from the axle housing to install a Lunchbox Locker, they are typically easier to install and less expensive than a Full-Carrier replacement locking differential.