Have you tried setting the temperature control to cool and leaving it like that? If that doesn't work your primary problem is likely the blend door. As mentioned it is a common problem with explorers and rangers. What's unusual is that you said you can make it stop by turning the defrost on and setting the AC? Do you have any other problems like always getting heat no matter what the temp setting?
The heater treater fix works great to replace a broken blend door, but if that's not the problem it obviously won't help. I had a broken blend door issue myself which gave me a similar issue, but until I bought the HT blend door I just cut a hole in the heater plenum (as if to install the new door) but just taped the old blend door to fully block the heater core in the summer and fully block the evaporator/outside air intake side during the winter.
At least on my 98, Max A/C will close off the flow of hot coolant to the heater core and prevent it from getting hot, thus preventing any heat from getting to the cab.