The hd offroad made the first versions of shackle relocators for retail and they where a lift style. They gave better onroad ride and better flex offroad. The two were only because of the angle of the shackle not the location of the mount. Dropping the mount does have some side effects like the one i talked about.
The no lift relocators give a better on road ride and better flex offroad while maintaining the propper spring geometry the jeep was design to have.
If you look at a stock xj setup you will notice the shackle sits at about a 45* angle to the road. Most manufacturers of lift springs make them the stock length with more arch. That pulles the rear spring eye forward. Changing the angle of the shackle to be more verticle. Add a longer shackle to the equation and that angle is further increased more vertically. The shackle relocator is adjustable to acomadate many different shackle and spring options and reset the shackle to the proper angle.