Another check would be to disconnect the hoses at the core and take one hose completely off the engine. Then using the hose that is still connected to the engine, put the other end onto the other outlet on the engine, making a loop using one hose. If that hose gets hot...you have a heater core problem. If the loop hose is not the same temp as the radiator hose, you have a circulation problem. Have not done this on 4.0 but have used it to troubleshoot V8's.
D30 w/powertrax no-slip,D35 w/ultimate 35 upgrade OX locker,4.56 gears,Air on Board, Mile Marker Winch
Warn Lockout Hubs