1. Turn Jeep off.
2. Turn ignition to 'On', then 'Off' three times within 5 seconds, but leaving it 'On' the last time.
3. The check engine light will flash two digit number codes to you, with a one second pause between the two digits, and a two second pause between each different code. A code of 55 marks the end of the error codes.
This is listed out in the Chilton's manual alone with a rough table describing what each code means, and is fairly common for mid 90's and up Chryslers.
2003 Dodge CTD Quad Cab 4x4, 320k miles and counting
98 ZJ Limited, 5.2L, NP242, Jet Stage II (worthless), K&N, 245/75-16 BFG KO's
89 XJ, stock
79 CJ7 - Options included ripped soft top, leaky 4.2L (5qts/week), ripped seats, 33" tires, bondo & gray primer