The thermostat is probably stuck in the open position.
this is not an answer just an update. coolant is filled
The first thing to check is the radiator fan.
Thermostat sticking. Replace thermostat and gasket.
pee on the engine it will cool down
I also have a 2000 jeep wrangler. This year has a problem with the heater core plugging up. You can either try to flush it out, or just replace the heater core.
push the lighter in, and when it pops out remove it. and remember IT WILL BE HOT!
Heater CORE, not coil. The core is plugged and needs back flushing or replacement.
blend door failure is a common problem on 97+ Jeep Cherokee
probably air in the system have you replaced the rad or hoses?
One of the reasons a 2000 Jeep Cherokee heater would randomly blow cold air and then hot air and then cold might be that there is a problem with the hot/cold duct. The air or a vent might be inadvertently blowing periodically.
Electric Hot Water Heater, Gas Hot Water Heater, Tankless Hot Water Heater, Hot Water Heater Leaking Hot Water Heater Problems Hot Water Heater Repair
Blend door probably broke inside the HVAC box.