I am assuming you mean the heater is not putting out hot air but the blower (fan) is working ok. First have the thermostat replaced then have the heater core flushed. If that don't do it, with the 4.0L engine, it's possible the water pump impeller is worn. Engine tempture in the normal range? Overheating, at all, on extended idle? (Sitting with engine idling for about 20 minutes) does the temp. gauge start to go up? Rev engine up after it's warm, does that help with the heater temp? If yes to any of those questions, have the cooling system flushed and water pump replaced! Verry common problems on this vehicle.