Jeep 2 letter designations, their years and a bit of history:

Quick list:

Grand Cherokee (2005+) WK

Grand Cherokee (1999-2004) WJ

Grand Cherokee (1993-98) ZJ

Cherokee (84-01) XJ

Wrangler (1942-1986) CJ

Wrangler (1987-1985) YJ

Wrangler (1997+) TJ

Liberty (2002+) KJ

Click here for modified jeeps of all types

Detailed list:

M170 - Similar to a military version of the CJ-6. 4155 of them were made in the mid-50's and early-60's.
CJ - The first civilian version of the military-inspired short-wheelbase Jeep, made '42-'86. Encompasses everything from flat-fenders to Scramblers. Like many other early Jeep designations, "CJ" actually stands for something: Civilian Jeep. Various numbers followed the CJ designation, indicating different body styles within the CJ line:
CJ-2A - A flat-fender, very similar to the military MB ('45-49) except that the headlights bulge out of the grill instead of being inset.
CJ-3A - A flat-fender, very similar to the military M38 ('48-53).
CJ-3B - A flat-fender, the top of its hood sits 4" higher above the fenders than the 3A to clear the new F-head engine ('52-68).
CJ-4 - Only one prototype is known to exist. It was discovered in 1997(?), still in the hands of its second owner. It looks like a 50/50 mix of a CJ-3A and a CJ-5, with only slightly-rounded fenders.
CJ-5 - The civilian version of the M38A1, but with the front shackles under the bumper like most other models. 81" ('55-'71) or 84" ('72-'83) wheelbase. Front fenders are rounded like all subsequent short-wheelbase Jeeps. Easily distinguished from the CJ-7 by a small doorway with a rounded, sloping rear edge.
CJ-6 - Essentially a CJ-5 that was stretched 20" ('58-75), intended for use as a small pickup.
CJ-7 - A compromise between the CJ-5 and CJ-6, having a 93.4" wheelbase (like the YJ and TJ). The body tub is very similar to the YJ and TJ, but the rear wheel wells are round instead of angular. Made '76-'86.
CJ-8 - Also called a "Scrambler." Similar to a CJ-7, but with an extra 10" of wheelbase and an even longer rear overhang. Like the CJ-6, it was intended as a small pickup, but never really found its niche. Only 27,000 were made from '81-'86. Commonly seen with half-cab tops that surround only the front seats.
CJ-10 - A more truck-like Jeep built mainly for export from 1981-85. Easily distinguished by the square headlights in the fenders and the 10-slat grille.
YJ - The original Wrangler, made '87-'95 (though production actually continued into March of '96). Rectangular headlights make it the ******* child of the of the short-wheelbase line. Rear wheel wells are larger and more angled than the CJ-7, but otherwise the two are visually quite similar.
TJ - The second generation Wrangler, made '97-present (actually began shipping May of '96). Round headlights, coil suspension, and a much more car-like interior distinguish it from the YJ.
SJ - The full-size Cherokee ('74-'83), Wagoneer ('63-'83), Grand Wagoneer ('84-'91), and J-truck ('63-'87). In 1984, Jeep introduced the down-sized XJ and started calling them Cherokees and Wagoneers, leaving only the Grand Wagoneer in the SJ lineup until its demise in '91.
M-715 - A 5/4-ton military pickup built '67-'69. It somewhat resembles a J-truck (SJ), only bigger and badder.
XJ - The down-sized Cherokee ('84-'01) and Wagoneer ('84-'89?). It was (probably?) the first Jeep to use a unibody chassis.
KJ - The Liberty ('02-present), which replaced the XJ in Jeep's product lineup.
ZJ - The Grand Cherokee ('93-'98), which replaced the Grand Wagoneer (SJ) as Jeep's luxury cruiser after a 1-year hiatus. Though much more aerodynamic than the SJ, it's obviously less rounded than its replacement, the WJ.
WJ - As the more bubble-shaped, aerodynamic replacement for the ZJ, it continues the Grand Cherokee name ('99-04).
WK - The third style of Grand Cherokee has a more angular design than the WJ and also joins the Liberty in having an independent front suspension ('05-present).
MJ - The Comanche pickup ('86-'92). It shares much in common with the XJ of that era, including its front unibody chassis. It replaced the full-size Gladiator and J-truck pickups of the SJ era.
DJ-3A - Dispatcher (postal) version of the CJ-3A, made '56-65. Only 2-wheel drive versions were available.
DJ-5 - Dispatcher (postal) version of the CJ-5, made '65-8? by Jeep and then AM General. Only 2-wheel drive versions were available. A DJ-6 (long, like the CJ-6) was also made from '65-68.
VJ - The Jeepster ('48-50) was an attempt by Willys to spice up their vehicle lineup. 2WD, 72hp, and poor marketing doomed it to be discontinued after less than three years.
C-101 - The Jeepster Commando ('66-73) was an attempt by Kaiser to compete with with the Bronco and TLC. The C101 was part CJ and part Wagoneer, advertised as a "happy combo, racy and rugged." This time around it had 4WD, and came in convertibles, hardtop wagons, and half-cab pickups.
FC - Forward Control pickups, with the cab directly above the engine. Made '57-'63