Be careful of using an infrared temperature gun for diagnostic work such as this. The emissivity (a measure of the efficiency in which a surface emits thermal energy) can vary greatly from one material to another and one color to another. Unless you have an infrared temperature gun that can be calibrated to a given material and color, the temperature readings you are getting could be way off.

Also, I have '87 Dakota with the 3.9L engine and although the temperature gauge always stays near the cold end of the gauge, the heat is good and warm even in the coldest northeast weather.