I'd get new springs - add a leaf is not going to fix the fact that the original springs are shot. Old Man Emu has great leaf packs and shocks. They are also meant for rigs that are loaded down, in case you want to do any camping.
I just want to add that a lot of what you do depends on your skill set, tools/space available, and how long your Jeep can be out of commission during modifications. There are a lot of guys on this forum with heeps of everything above - they have Jeeps that are not DD, garage space to work on them, and skills that really humble you. I am amazed at what they can do, and continually learn from their posts. However, if you either cannot go a few days-weeks without a functional vehicle, lack the skills or budget to accomplish them, then some of the more complicated modifications might be out of your reach. For example, if you go more than 3" you are going to need a slip yoke eliminator (SYE - shortens the bit coming out of the transfer case to decrease the driveshaft to rear differential angle) and a new driveshaft. Add to that regearing (much advised), and you have some significant time and money investments.
I am saying all of this because while I have had 4x4s my whole life, I did not have any experience going into my XJ project. My biggest advice is to keep in mind that once you make one modification, a chain of events is set in place that will require more modifications. I started where you were a year ago with a stock, saggy rear end XJ. A year later and about $5500 in quality parts, I have exactly the rig I wanted.