Welcome to the forum!
Typically when the blower fan works only on high speed, that's a sign that the blower motor resistor is bad or the connection to the blower motor resistor is bad. Chrysler products of that timeframe did tend to have issues with blower motor connectors that melted and therefore made poor contact. You might confirm that the connection to the blower motor resistor is good. Since you replaced the resistor and it worked for a bit further brings the connector in to question.
You only mention that the rear seems to have a heat issue. If the heat is fine in the front when the blower is working, the problem will not be due to a thermostat. That is probably more of an issue with either a failing blend door or some other system that either restricts flow of coolant to the rear heater core, a plugged rear heater core or passage, or something that allows too much unheated air to mix with the air flow. Has the rear heat worked at some point since you've owned the Durango? I've head of folks just looping the coolant lines to the rear due to corrosion or other damage, and if that was done on your Durango it would certainly prevent you from getting heat to the rear.
-Rod