I've done it before. You need a new diff head. Can't just add the locker part. As mentioned the axle housing may have to be notched, not really a big deal, just need a torch and grinder. You will also need a new axle shaft as the locking one is double splined.

Get the model number of your diff and look for used or reman units in your ratio with a locker. Going to spend around 3 to 6K to do both axles depending if you go used or reman.