Viscous Fan's are actually easy to diagnose!
From a cold start, have someone start the car, the fan should 'roar' (ie engage) before it quickly quietens when the Viscous Clutch disengages because of the engine temp.
If it's heating up at highway speeds, it will be a Radiator/other issue. If you have actually wheeled your Jeep through a bit of mud, between the Radiator and the Condenser (and if it's a CRD, the Intercooler) there's heaps of places to catch heaps of gunk. It won't come out cleaning the radiator while it's in the car or hosing it from the front. You will have to use a fine spray and try to wash it out from within the engine bay (reverse) to get it clean. If there's still crap in there, you will have to take it out. From experience, these Radiators aren't too bad to clean from within the Engine Bay. For me, it involves lying UNDER the Jeep with the hose as mentioned above getting covered in Mud and Water (and considering Tassie's temperatures, colder and Colder).
If it's getting hot at Highway Speeds, it won't be a Thermo Issue, Thermo's will have no real additional cooling effect at highway speeds.
If it's at low speed only, get the Thermo's checked. A quick way, if it has an Electric Thermo, is idle it in the driveway, rev it a little but basically sit around and wait, if the Temp Needle rises and a Thermo does not kick in, you have an issue (or if it kicks in but not really fast etc etc).
Oh, and I had a mate who put Electric Thermo's in his GQ, when wiring it up he reversed the polarity of the Fans so they sucked (from the engine bay) rather than blew into the engine bay. He had all these issues with cooling and he couldn't work it out for ages until we realised this! If someone has been stuffing with your fans, this may be worth a check (however it would be very unlikely!)
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