O.P. has a Rubicon which all come factory equipped with discs. @inkedrose , not mentioned is that the Rubicon rear axle uses different shaft lengths between left and right.
Mine is not a Rubicon and the shaft lengths are different. I remember discovering that after I'd pulled them and had to figure out which one was supposed to be longer. A simple test fit didn't work since I couldn't see into the locker case. I can't remember now but I kept them separate and matched them up when I installed the RGAs.