There have been several threads on the subject recently. I added cruise to my '99 XJ using a factory parts kit purchased online from Jeepsareus.com. Others have managed to get what they need from salvage yards. You will need the following:
*Servo unit w/throttle cable (complete assembly, remember to snag the vacuum hoses as well).
*Clockspring (located behind steering wheel, the one for cruise units has an extra wire).
*Left and Right cruise switches ('97-'98 and '99-01 are different SKU's, be sure to get the latter). You can re-use your existing steering wheel.
*Y-cable that goes between the switches and the clockspring.
I installed mine in about an hour or so. The hardest part was figuring out how the clockspring came out of the steering column. It's easier once you have the new part in hand and can see where the mounting tabs are located. I also had to open the servo mounting holes in the engine compartment up a little with a drill, as the screws in the kit were too large.