I'm one of the people who say extended shackles cause spring sag. Others don't agree.
Here's how I see it working:
The XJ is a spring-over-axle configuration. That means at rest and unloaded the spring sits more or less flat. Under motion, when you crest a bump and unload the suspension, the spring curves up and when you load the suspension the spring curves (bends) down. The downward motion stops when the axle hits the bump stops. Remember, the suspension pivots around the front spring eye, which is fixed. The front half of the spring is stiffer than the rear half, so if the axle moves up, for example, three inches to hit the bump stop, the front half of the spring bends a bit but also pivots, while the rear half of the spring bends (deflects) downward about 5 inches.
Now fast forward to the same spring with a shackle that's 4" longer, for a 2" lift. Why a 4" shackle for a 2" lift? Because of geometry -- the spring and axle rotate around the front spring eye, which is fixed.
So, now we again load our XJ until the axle hits the bump stops. No extension ... bump stops remain as before. So the axle moves 2" farther up (or the chassis moves 2" farther down, whichever way is easier for you to visualize it). The front section of the spring is still stiffer than the rear section, plus the whole assembly pivots around the front eye, so the forward part of the spring doesn't deflect much more than it did before. But now look at the rear. The chassis is down on the bump stops, the front spring eye is in essentially the same relationship to the axle as it was when loaded under stock conditions ... but now the rear tip of the spring is 4" lower than it was with the stock suspension. Most of that difference is taken up by the spring bending to the rear of the axle. That's a lot more movement that it was designed for, and after enough of that the steel begins to fatigue because it is moving far enough to exceed its elastic limit and go into plastic deformation. Plastic deformation is non-recoverable.
At least, that's my theory. It makes sense to me, but as I saw in someone's sig on some forum, "In theory there's no difference between practice and theory. In practice, there is."
