Although a few people have discussed ... in theory ... the notion of making new spring mounts, if anyone has actually done it I don't recall seeing anything posted here to describe it. Frankly, not everyone feels like taking on the work and the risk of meddling with something as fundamental to safety as the way the rear suspension attaches to the vehicle.
I see some practical difficulties with what you propose. Let's say you move the forward spring mount forward and down by 2 inches, which is what I think you used as an example. Then let's say you find a spring pack from some vehicle that's about the right length and arch to fit. But you've dropped the front mount by 2 inches, which has the effect of pushing the pinion angle down by several degrees. The lets say to get the axle back to where it belongs in the center of the wheelwell you have to re-drill the spring pack and move the axle back several inches. Now it's not at the level part of the spring, it's well back on the sloped portion. You've just nosed the pinion down a few more degrees. Sure, then you can cut the mounts off the axle and re-weld them, but now you've created still more work for yourself.
Most of the folks who do big lifts on the XJ don't retain the stock main leaf, they buy complete spring packs. Those who retain the main leaf do so for one or both of two reasons: (1) cost -- an AAL is a lot cheaper than a spring pack; and (2) labor factor -- especially with an older XJ in the rust belt, it can be a nightmare to remove the OEM spring bolts without busting loose a weld nut or encountering other problems. If the person only wants a modest lift, all those problems are avoided by leaving the main leaf attached to the chassis and installing an AAL.
If you'd like to be the rolling test bed for a relocated spring mount, I'm sure many people here would be interested in seeing the results and reading about the process.
