In the last month or so, the heat and air conditioning only work when
it is set on high. Any suggestions?

Sounds like a bad blower motor resistor.

Thank you Jpb53.

I found this helpful:

http://www.gtrippleb.com/auto/durango/tech/durango_blower_resistor.htm

Does anyone know if the location of the resistor is in the same for 1998 Durango?

would this effect air not being blown out of the center set of vents as well? my front blower just started not blowing except on high on my 2001 durango, my middle vents don't blow at all, and the back vents blow on medium and high but not on low... sounds like this resister will fix the front blower, but what about the middle/back?