wow, that is a lot of spacer. I'm just guessing that the 1.5" lift you already have is also spacers as well. I would stay away from stacking them like that, but don't know for sure if it is doable.
I do know that anything over 3" on an xj is going to cause problems with the rear driveline. You run the risk of over extending causing the slip yoke to pop out when flexing, as well as u-joint binding. I personally ran into vibration problems at 3 inches of lift on my xj before lowering the t-case about 1/2 inch.
If you do go offroad a lot, the add-a-leaf will also wear your rear springs severely, so you will eventually have to replace them.