highdesertranger said:"but a locking diff has to be engaged with a switch or button on the dash"
not true.
Ok...I did say it was the barebones simple answer (that makes sense in this context)...not trying to cover ALL products of every manufacturer, choice, and design.
For the OP, it is probably 99% certain that if he walks into a shop and says I need a locking diff installed into this (road legal) vehicle's rear end, they will be installing a switch on the dash for the electric or air operated mechanism. Most people who install a locking diff are doing it BECAUSE they can switch it on and off as needed.