The difference is the height of the upper part, I'll call it the 'H' part of the shackle. It is longer in the rear because it is mounted in the middle of the framerail and the stop has to clear the bottom of the frame. In the front it is mounted on the bottom of the framerail so it is shorter. I am assuming to get the same amount of droop at each end.
Mine cost about $40 in material. I believe Jeep Thrills has a pretty good price, but I haven't checked around.