Having Same Problem
I own a 2003 F150 SuperCrew 4X4 FX4. I recently started having the same problem. I only get air to the defrost ducts in PANEL, PANEL FLOOR, DEFROST FLOOR, and FLOOR modes. I can get air from the panel by pressing my VENT botton. I'm not sure where to even start to fix this problem. It sounds as if we are both having the exact same problem! I look forward to any help in the way of advice and any diagrams and schematics related to this issue.
Trace in Indiana