Axle wrap is caused by torque from the driveline trying to twist the rear end, basically the pinion rotating upward. The springs warp, basically turning into the shape of an S, then the spring force resists and forces the spring back, then the torque pushes it up, the spring forces it back......on and on. This dramatically shortens the life of the springs, causes a loss of traction as the rear end usually hops, and if the pinion raises enough it will snap the pinion yoke.
Blocks increase the leverage that the axle puts on the springs, thereby increasing the tendancy to have spring wrap. Big trucks get away with tall blocks because they have stiff springs which are designed to carrry a load, and those stiff springs can successfully resist wrapping. If you have stiff enough springs to not have axle wrap on an XJ with 3-4" of blocks, you have one stiff riding, non-flexing rig......neither are a good thing. Even 2" blocks can be a proplem with a good set of springs, unless the springs are designed to resist wrap or you use a traction bar. Look at the other thread currently going on regarding axle wrap.
The taper has nothing to do with whether axle wrap will be caused. The taper merely points the pinion up slightly to improve the driveline angle. I would not recomend using 3" blocks.