Most common reason for the clucth not engaging is low freon pressure - a system leak. Look at the pilot/sight glass on the AC radiator coil (in front/Left of the engine radiator coil) - make sure that there are NO bubbles/foam visible in the sight glass when the AC compressor is running - if bubbles/foam, then you have a leak. If the pressure is low (bubbles visible) the switch on the side of the sight glass wont energize the compressor clutch circuit. If low pressure and you dont believe what you see in the sight glass, with the engine running ... momentarily short across the two prongs of the pressure switch connector, if the clutch engages then you have either leaked freon to too a low pressure to operate the switch/clutch .... OR the switch is bad (very rare). First, make sure the pressure switch connector is not loose - clean it free of corrosion and make sure it's TIGHT. Same for the thermocouple in the AC air outlet panel in the heater/AC duct under the dash. There is also an electrical connector on the AC compressor (about 6 inches of lead wire, then the connector) - make sure that it's not loose.
Next time you refill with freon, add an appropriate dye so that any leakage can be easily found (by blacklight). If you do have a leak, you can add a leak-stop compound/fluid to the freon .... as a first step before you go rippin' and replacin' the AC system components.
If the system has leaked, and already contains R134a refrigerant, you can recharge the system yourself by buying cans of R134a freon and a freon 'pigtail' at just about any auto parts store. I dont know what year they began to use R134a refrigerant in Jeeps, perhaps someone else on this NG can help on this. DO NOT add R134a until you **KNOW** what's in the system, as the different refrigerants used in automotive systems DONT MIX and MUST be compatible with the lube oil inside the compressor. For myself, when refilling the AC system I keep adding cans of freon until the airconditioner outlet at the dash reads about 40 degrees F.
:-)