If the rear is a lot lower than it should be, you may have broken a leaf. The 2WD and 4WD XJs use the same springs, so you can use any XJ springs, from any year. You can also pull additional leaves from another set of springs and splice them into your spring packs.
First, though, be sure it's really lower than it should be. If you still have the flares, measure stright up from the center of the axle/wheel hub/wheel cap to the bottom edge of the flare. For a stock XJ this dimension should be 17.0 inches.
If you don't have the flares, a month or so ago I posted the measurements from the axle tubes to the frame, per the factory service manual. Someone else added the diagrams. I saw a post you had yesterday asking about "flat" springs, but the link didn't work for me. In general, most people aren't accustomed to spring-over-axle setups. The stock springs always sit just about flat, but folks expect leaf springs to have an arch because they are expecting them to look like springs that run under the axle.
Always measure before declaring that your XJ has sagged.