Any time a heater core is found to be plugged up, it’s a sign that the cooling system has not been well-maintained somewhere in the vehicle’s past. That, in turn, indicates that the radiator is, at the very least, not in OEM condition (unless it was previously replaced due to blockage.) if I see a plugged heater core, I immediately assume the radiator is in need of replacement.
The XJ and MJ series of vehicles are NOT tolerant of cooling systems that are in any condition except like new in terms of performance, as the design has VERY little margin for deterioration.