What I did on a couple of Jeeps with sagging springs, to return them to stock or a tiny bit higher, was to add a third, rather than a main leaf. If you look at the springs and the way they are stepped when stock, you will notice that there is a huge gap in length between the second and third springs. If you take a main or second leaf, you can cut it so that its length falls neatly between the lengths of #2 and #3. It becomes the new #3. It adds stiffness and arch to the pack, and of course its own thickness, but keeps height near stock, and has no appreciable effect on ride, except that you won't lose so many teeth when the springs hit the bump stops. A single leaf does not require new U-bolts. You'll have to make or get new clips, and a longer center bolt.
If you have any VW fans in your neighborhood, try to get a couple of the spline or allen head bolts from the U-joints. Just about every VW from the 68 Kombi at least well into the Golfs and Jettas of the 90's uses the same size. They're exactly the right center bolts for an XJ with one extra leaf. The heads are already rounded, they're hardened, and they even take the same nut.
