Hvac won't go to defrost or to floor . . . stuck on vents. Temperature control is fine

You were right. I did a little research and it's a very common problem. I went ahead and tore the dash apart and replaced all doors contained in the heater box. While I had the heater box out I replaced the heater core and AC evaporator. Being 12 yrs old there is no reason not to replace them. It took a total of 8-10 hours at a leisure pace to complete the entire job.