Air bags or shocks would work OK if the added load is occasional. They would allow you to adjust the ride height for your changing load.
New HD springs would be best if you are constantly carrying the added load. Stock XJ rear springs are weak, and I would recommend new units with a "military wrap". Deaver makes nice springs, but they aren't cheap because they custom make them for your application and exact weight. (I'm lucky, they are only about 1 mile from my house!) Maybe an off the shelf set listed for about 2" lift would add the needed capacity? It might be worth an inquiry or two to some of the Jeep lift kit manufacturers.
Another option is to use add-a-leafs. Here's a link to an article on this website that explains it pretty well:
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