Supposedly, "XJ" stands for "eXperimental Jeep", but that is probably apocrypha. Most of the "nJ" designators son't seem to mean much of anything, although "FSJ" is thought to mean "Full-Size Jeep", and the "J-Series" truck was probably an answer to the "F-Series" by Ford and the "C/K-Series" from Chevvy. Apart from that, it's anyone's guess. I don't remember seeing anything that really answers this in any of my Jeep books anywhere...
Lemme guess - clear as mud now, right?
5-90