The Stock XJ leafs are comparatively "flat" in shape compared to aftermarket lift leafs that have a greater arch. And the stock shackle looks to be about ~4" ?
The leaf absorbs shock by "straightening " or flattening. The front of the leaf is firmly attached to the body, so as the spring flattens the rear of the spring moves back, and the shackle swinging back allows this.
The more arch the spring initially has, the more the rear eye needs to swing back. A stock, almost flat leaf doesn't move much so the short stock shackle is fine.
with an aftermarket leaf with more arch, the rear of the spring move back much more, and I would think the stock (short) shackle is going to limit this, making the leaf "bottom out" sooner than it should?

So that brings me to a few questions:
1) Why do people want a 45deg shackle position at rest? you are already giving away 1/2 the travel of the shackle,, would not 90deg (straight down) give you the most travel?
2) is not it more important to have a Longer shackle with aftermarket springs, so you have more travel before the spring is limited by the shackle swinging straight back?
3) do leaf spring manufactures state a min shackle length?
4) most shackle mount relocation brackets drop the mounting holes down 3-4" from stock,, plus using a longer aftermarket shackle, doenst this lift the rear a few inches?

Let me know where I am going wrong here?
Getting ready to address the rear suspension and mounts, while I have the rear all opened up (rear floor pan removed) and welding in there.

Slack

OK, question on after market relocation brackets:
I see the are typ 1/4" thick material, does that "flex" enough that when you tighten down on the shackle bolt it "squeezes" the shackle bushing and holds it?
or is the shackle bushing inner sleave just rotating on the shackle bolt? creating wear of the bolt and inner sleave?

Slack

1) having a static angle of 90 would give you the most potential uptravel, however it completely removes opportunity for down travel except for decompression of the leaf. Also at 90 you have maximum force transfer into the vehicle, at 45 the force is evenly distributed up and back
2)A longer shackle does allow for more travel
3)I don't know for sure, the spring will behave more optimally with a longer shackle but it will still work with a short one. There probably is a sweet spot but I doubt most manufacturers really put in the R&D to figure it out, maybe look at Jeepspeed style rigs and copy what they do.
4) yes it does raise the rear, sort of, if you have a 90 degree shackle, kicking it back to 45 degrees makes it's effective overall length in the "y" plane shorter, we'll call this "S", equal to S*cos45, for example a 6 inch shackle at 90 degrees is only 4.24" tall sitting at 45, so while the bracket drops the hole a few inches, the difference in shackle angle could compensate (might want to double check my trig, but concept is legit). Also since you have your rear opened up, check out the no lift relocation brackets by HD offroad engineerning, this way you can use a longer shackle to optimize travel, without getting too much lift.
5)the brackets I have (HD offroad standard) did not have this as an issue. 1/4" steel will flex enough, but in my case it was a very tight fit to begin with.

1.) 45° provides for "equal" up and down travel (thought many other factors could limit this). It also provides (in general) the best ride without slamming your shackles against the hangers.
2.) No, it is not.
3.) No, they do not, there is no reason for them to.
4.) Depending on length of shackle and angle yes and no.

Thanks for the response,, I have read more about leafs, shackles and relocation brackets in the last 48 hous than I care to admit.

Cut and ground off the stock shackle mounts and picked up a bunch of flat steel to fab up new multi position weld in brackets.

Slack