Hey good question and Zed got it right. I was asking the same question to Toyota as well and they just said "it improves my offroad performance" also if you look in your truck manual they state that you shouldn't go over 5 mph when it is locked. Thats a load of bull. I contacted a Toyota Mechanic and asked him: "what will happen if I have the rear wheel differential lock engaged and go offroading with it?" "Larry" said that nothing bad will happen. Although he did add one caveat to that. As long as one of your tires can slip when making high speed corners then all is good. He also recommended that I do not use this for normal driving on the pavement. The hardball will not allow for one tire to slip which could potentially snap the ring and pinion gears or at the very least snap the diff lock pin and have it tumble around in the "pumkin". I dive with it engaged when I am driving offroad. I had it up to 50 while cornering. Nothing happened. I was on a dirt/gravel road. Hope this helps.