The 4Runner comes stock with electronic traction control. It's OK for mild situations, but when the going is really tough it can't quite deliver. Deep sand confuses the TRAC system, and it's impossible to get the needed momentum when TRAC keeps cutting the throttle and pulsing the brakes. We wanted a bona fide locker instead. We wanted said locker to be automatic and self-contained. These criteria led us to the tried-and-proven Detroit Locker by Eaton Performance.