The axle u-joints have to be aligned with the ball-joints so that there is no binding when the knuckles are turned, and also so that the inner shaft is the correct depth in the pumpkin. Since the outer axle shaft is bolted to the hub unit, the outside edge of the hub unit bearing is what determines the position of the shaft. The WJ and XJ/ZJ/TJ hub units are different in this regard. The purpose of the spacer is to reposition the older-style hub unit on the knuckle so that the final depth of the axle is correct (although you have to use the 00-01 XJ hubs). You can use the WJ hub unit without a spacer, and the final position of the axle will be correct for the knuckle, but it will have 5x5 bolt pattern.
I've done some preliminary research into doing a WJ 5x5 conversion for the rear but have not figured out if it's feasible. The flange on the WJ rear axle is square, as is the bearing retainer plate, and all of the other parts (such as the disc brake backing plate) also make assumptions about that same basic shape. However the XJ, ZJ and TJ all used other shapes and bolt patterns for the axle flange and bearing retainers. In order to bolt the WJ rears onto a ZJ axle you would need to either replace the flange on the axle, or fabricate some bearing retainers that used the WJ square shape with the ZJ rectangular bolt pattern. At some point you would also need to drill and stud your ZJ axle shafts to have a 5x5 lug pattern so that you could use the WJ rotors. [edit--you will also need to verify the distance from flange to rotor is the same as the stock setup so that the axle depth in the pumpkin is correct, and you may need to fabricate spacers to get it right.] Somebody on mallcrawlin put WJ brakes on a Ford 9" axle so this is all technically possible, but nobody has done it for XJ/ZJ/TJ axles that I know of. The difference in the ZJ and WJ rear disc brakes is very small, so most people think it's a waste of time.