So, you're adding the Dakota leaf pack to your XJ main leaf? I believe the Dakota pack (main leaf) is longer than the XJ, so you'd have to move the front spring hanger forward to use the whole Dakota pack.
Here's some general info on making the rear leaf pack work well. To show that I know what I'm talking about, I have only used home built rear leaf packs, and here's a couple of pics to show the balance between front and rear. This pic is with stock length leaves in the back and RE ZJ coils in the front.
To get a leaf spring pack to ride well and flex, the friction in the pack needs to be minimized. Make sure the shackle bolts and front spring bolt aren't too tight, you should be able to turn them with an end wrench.....snug, but not tight. There is considerable friction between the leaves, especially after they get rusted and gunked up. Go to any local spring shop and get plastic liner and use it full length between all the leaves. Wire brush and sand down each leaf and paint it with a good quality paint, I use an epoxy paint that comes in a spray can that is very tough. If you end up short on liner, or can't find any, at least clean and paint the leaves, use a plastic end pad on every leaf, and use a piece of plastic in the center of each leave about as long as the spring plate. This will allow a gap between each leaf after the u-bolts get tightened down and will reduce friction, but the gap can get filled up with mud and still rust eventually, but will be better than the leaves actually touching each other. It's still smoother with the full length liners if you can do it. There are two types of liner, one is flat, and the other has small edges on it like an I beam. The flat will work itself out between the leaves, so I only use it in the middle leaves that have the spring clamps to hold them in. Between the lower and upper leaves I use the stuff with the edge on it so it stays put. Take a cold chisel and notch the edge of the liner for the u-bolts before you install the spring packs.
Generally, more leaves and flatter leaves will give you better flex. The flatter springs will droop more, since a pack with higher arch is already arched, not much more to give, and a flatter spring can droop (arch) a bunch. People talk bad about blocks, but I like them. I build the leaf pack to handle the basic lift and load, but not too stiff to ride nice and flex, and then level out the rig for desired lift with blocks. This approach requires a traction bar, since flatter leaves with less friction along with blocks will increase the potential for spring wrap, which will destroy a good leaf pack in a hurry.
The other thing is to have loose spring clamps. You need leaves that have the U shaped clamp that is riveted to the leaf, with a roller on the top. If you have the U shaped clamps that are riveted on the bottom, but bent over at the top, straighten them out. You need spring clamps to keep the leaves aligned, but you don't want the clamp to squeeze the spring together or limit them sliding. You can run the clamps open on the top if you have a lighter rear end and smaller tires, and a good traction bar, but you are risking breaking a main leaf. With larger tires and a heavy rear end the main leaf will drop too much and decrease both vehicle performance and spring life. The best is to keep a bolt through the top of the clamps, across the spring, but lengthen them enough to allow good movement of the main leaf in droop.....but not too much.
It doesn't much matter what leaves you use, out of what vehicle, or how much arch they have. Once the center bolt is tightened, they all come together and each leaf adds it's force to the pack. Just be sure to space out the length of each leaf as evenly as possible, trimming the length of a leaf if neccessary. This is where the full length liners come in handy because you don't need to worry about cutting off the hole in the end of the leaf that holds the plastic friction pad. It will take some trial and error to get it the way you want it, but removing springs isn't hard. Get a supply of center bolts, and while you're at it get some new u-bolts, before you start. If you run a big rear end with big tires, it's also wise to increase the size of the u-bolts from 1/2" to 9/16". Any local spring shop can whip you up a set of u-bolts while you wait.
So, loose shackle bolts, loose spring clamps, full length liners, and relatively flat spings, and you'll have a spring that can carry weight but still flex and ride good. My current springs are 1/2 ton Chevy with the stock XJ bottom leaf, and an MJ main leaf, which is five inches longer than the XJ main leaf. I also seem to always have one or two leaves more in the pass side pack than the drivers side pack, which I think comes from the torque loads of low gears, and possibly from the action of the traction bar, or from my old swing out tire carrier sitting open in camp.......don't know for sure, but there is nothing wrong with having more leaves in one side than the other if that's what it takes to even out the lift height side to side.
Hope this helps, have fun,
