Although your mechanical fan is always turning it has whats commonly referred to as a clutch that engages when the temperature gets to a certain point. This "locks in" the fan to turn at the same speed as the engine to rapidly cool it down. When the temperature drops the clutch disengages and the fan speed will slow down. What can happen is either the bearings in the clutch will seize up and your fan will be turning at engine speed all the time which will not allow your jeep to run at normal operating temperature or the clutch itself basically stops working (sounds like your problem) and the fan will not engage to cool down your engine. It is actually a pretty straight forward repair that I'm almost certain you could do yourself. An easy way to test the fan clutch is to rotate it by hand (engine off), you should feel a slight resistance when you turn the fan, if it freely turns its no good, if it doesn't turn at all it's no good, if you can rock it up and down it's no good. If you don't already own one I suggest getting a Haynes or Chilton manual for your Jeep, it will walk you through most of the repairs you will have to do on the Jeep including the clutch fan replacement.