Actually, the Jeep "X" is the base model Wrangler, followed by the Sport, Sahara, and Rubicon models (I choose not to include the Renegade model). The easier way to understand Jeep models is to compare by year of the vehicle, not by the names:
http://en.wikipedia.org/wiki/Jeep
Willys is the designated name for the military Jeeps everyone is familiar with (i.e. Olive Drab, flat-fendered go-everywhere military Jeeps). After that, you have the CJ (Civilian Jeeps) from 1944 to 1986, followed by the YJs from 1987 - 1995, the TJs from 1996 - 2006, and the JK from 2007 on. In the middle of that you have your FSJs (full-sized Jeeps), your MJs (Comanche pickup), XJs (Cherokees from 1984 to 2001), XKs (Commander), ZJs (Grand Cherokees from 1993 - 1998), WJs (Grands from 1999 - 2004), WKs (Grands from 2005 on), KJs (Liberty from 2002 - 2007), KKs (Liberty from 2007 on), and the MK line (Compass and Patriot). There are others in the mix but they are all listed on the Wiki link I gave you.
As far as model kits go? Not many. I've only found the '77 Jeep CJ Revell kit, the "Daisy Duke" CJ, and the Lindberg Super-Snap ZJ Grand Cherokee out there, although the Jeep Honcho (pickup) was also available at some point. The Jeep line is woefully restrictive on their models and as such, no one seems to want to release them freely to the modeller's world. This is VERY UNFORTUNATE, since there are many offroaders that would love to see their rock-crawler/grocery-getter in 1/24 scale.