The three times I've encountered this problem, it's the thermostat failing to open, stuck closed, not allowing coolant to flow... Be sure the coolant is at proper levels, all air has been purged from the system and there's no leaks in the system... Does the upper radiator hose get hot with the engine running, it should if there's coolant flowing through it when the engine is up to temp... Possible culprits: thermostat, water pump, system leak, clog I'd replace the thermostat and flush the system a few times, to start, if there's nothing else obvious going on...
'94 Limited ZJ (CRUSHED, kept the 5.2L, 46RH & 249 TC)
'96 Orvis ZJ
'98 Slate 5.9 ZJ (DD)