I have a 2003 sierra 1500 4wd ext cab

My heat cannot be adjusted, if it is set at 90 I get hot air. If its set at 89 or less I get cold air. No in between. Does the same thing with the A/C settings too. Doesn't matter if it is set to auto or not - same either way. I can adjust the heat for each side of the truck, each side does this - I can have hot air on one side and cold on the other.....

I searched the forums, but dont think I have found the answer, but I have it narrowed to a few things:

1) the blend door mechanism is not working.

but, I can feel the thing turning when I adjust the temp settings. I read a gear in there can break/strip causing this?

2) it is a bad temp sensor above the driver.

Only way to fix this is by the dealer since it needs to be set according to the VIN number??

Any ideas on which route to take? I don't want to spend the @ $140 for a new blend door unit if this probably isnt it, and since I really dont have a clue what it actually does it makes it tough.....