If I'm reading the description of the coils right, it sounds as if they may only offer overload boost without increasing unloaded height. So they may help with scraping tires, but not with the initial sag. You will at least need to measure the length of the coils to see how they'll fit when unloaded.
I don't know how the coils would work, but as an alternative, if you just want to restore height and stop hitting the bump stops, you might search here for threads dealing with adding a leaf. Especially if you can get hold of another old set of XJ leaves, perhaps from someone who has tossed them for a lift, it's really easy to cut down a leaf and add it to the pack. The U-bolts can take one more leaf in thickness, so all you need is a new center bolt and clips. One more leaf should put you pretty well back to stock height even if your springs have sagged a good deal.
Here's what I've done with both my current saggy old Jeeps, with good results. I got more or less back to stock height, firmed up the ride on bumps, without changing the basic ride much. Others have gotten similar results just cutting the eyes off a main leaf and putting it as leaf #2 beneath the main, but I thought this a bit neater, and it removes some of the negative arch.
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