open vs limited slip diferentials.

I would recommend you get a factory limited slip axle on your 4x4. Air lockers are expensive, and can be less reliable than the factory limited slip units. You mention adding an air locker to a vehicle that already has a limited slip differential. That would not be necessary, as they both achieve the same purpose. Unless you are talking about adding the air locker to the front differential, which would not be needed, as the vehicle will not be used for extreme off roading, i.e. rock climbing, etc.