X vs Sport

In all honesty it was probably called to X because of AMC tradition. I believe the first "X" car was the AMC gremlin X, the X style package added a cool stripe to the AMC. The wrangler X added a stripe as well
The Gremlin “X” had a V8 with an auto or a 4 speed.
 
Every TJ 03-06 had rear disk. Every TJ 97-02 has rear drum.
I have a 2006 TJ Sport and it has rear drums. As a matter of fact I am considering selling my Jeep to purchase another 2006 Tj Sport with all around disk brakes and abs.