I have a 2002 Dodge Ram 1500 Quad Cab with a 4.7L V8 engine in it.

I've replaced the thermostat, rad, water pump and heater core. I get fantastic heat from the passenger side dash vents, but the driver's side is much less heat (still warm) and if I drive the truck for longer than 15 minutes at highway speeds, my heat on the driver's side and the defrost decreases from warm to lukewarm.

I've checked the door, rod and actuator under the glovebox area and that seems to be both intact and working perfectly fine.

What else could cause me to lose heat? PLEASE HELP!

 

Main issue is the way they constructed the doors for your hvac unit in your dash. You can learn more from here:
http://heatertreater.net/dodge-ram

You can buy their stuff, or OEM if still available... I bought a Dorman brand replacement in the past...

 

The blend door can break in half and only let one side work from my understanding cant see it unless removed ,I pulled my dash and replaced all 5 blend doors , ,,pulling the dash isn't really that hard but you will lose your freon , if you can recharge it yourself its not bad at all , of course it depends on your mechanical skills and replace all 5 blend doors and heater core and condenser while your in there if you decide to tackle it