What do you use your Jeep for?
Why do you think you need a locker?
Theres different types of lockers and each one acts different on and off road.
Automatic lockers(Detroit), full spool, mini spool, weld, selectable (ARB, Eton E, OX), Limited Slip, and ratchet style drop in lockers (Spartan, Aussie).
As far as install goes. Full case lockers (Selectable, Automatice, LSD) will require you to reset up your gears. This can be very costly depending on the shop. And shouldn't be attempted with out the proper tools, and knowlage.
A cheap route would be the ratchet style lockers like Aussie, Spartan, Lockrite. These replace your spider gears with a ratch style. Install can be easy and done with common hand tools. These can be loud and sometimes unpredictable on road. As they lock and unlock as you go around corners, and release and apply torque.