Locking Differentail

I have a 2000 Jeep TJ and I wanted to get the Locking differentail but I didn't know if i could just buy a part that will allow me to modify the differentail that currently have. I do currently still have stock tires on and stock ride hight but I do plan on getting a 6-8 inch lift kit and putting on 36's. Also do I have to put the lock differentail in both the front and back or will the back be enought.



Thanks