If it does go to the front diff, most likely someone put in a manual disconnect for the front diff instead of running with the vacuum diaphram actuator to lock it in...

 

hey im sorry it goes to the REAR differential

 

Well I am just guessing but it might be some type of manual locking mechanism for the rear axle....Duh

The only way to find out is to either try it out or take it to a tranny/rear end shop to see if they can explain it to you.

 

what would locking it do? why and when do i need to do this?

 

locking the diff. just means that youre locking the gears inside together so both of the tires will spin in a low traction situation instead of just one tire spinning like with an open diff. if its engaged and youre on dry pavement and you hear the tires chirping when you turn, its a locker. if the tires dont chirp then more than likely its not a locker, or its not engaging properly.