put a add-a-leaf in
helps the old leafs out also
It will make your ride stiffer but will get rid of the sag.
You might think your XJ is sagging in the back but it's might just be the wheel wells that are higher in the front. Check it out. Mesure the height from the body line not from the wheel well.
I opted for the shakle myself. I didn't want to give up the little flexibility I had in my rear leaf springs.
I added in a 2" shakle in the rear and a 1.75" spacer in the front on my 3" lift kit. You end up having close to five inch lift for not too much extra $$$. You will have to change your front shocks for a 3"-6" travel ones (the rear one will be ok). I didn't drop my T-case either and I just had a little drive shaft vibration. It was bearly noticable.
That's what I did .. i hope it helps (I have a 99XJ).
Cheers,