Looking good, I like the clearanced axle. Are you staying spring over or going spring under? That alone would probably give you right about 5" out back. I'd imagine the leafs from a Dakota or S10 added in would help with spring rate since they're designed to carry a heavier load than XJ leafs.
Spring under a Ford 8.8 with LSD, disks and 3.73 gearing. With the SU and higher rate leaf packs we are hoping to be around the same 3.5" drop as the front.