Rusty's Offroad has spacers for $38 a pair. I'd use an full-length add-a-leaf to lift the rear of an XJ, doesn't change the pinion angle as drastically as shackles. You're right in that shackles give half the lift of the difference in length. A 4" longer shackle would only give 2" of lift. You'd have to drop both ends of the leaf spring 4" to lift 4".
Rusty's also has 3" front XJ coils for $99, that's what I did. I then bought MJ (Commanche) shackles that lifted the rear 1" and bought 2" lift blocks from
JC Whitney for 3" total in the rear. I would avoid shackles, and lift blocks increase spring wrap and wear out leaf springs quicker. Like I said, go with full-length AAL's for the rear of the XJ. I'm pretty sure Rusty has those too.
You can even add a couple of stock coil spring isolators (3/4" each) to the 2" spacer for as much lift as you want, they are safely stackable.
Give a shout if you have any qustions.[addsig]