I have repaired the blend door with a crafty repair I found on another
site.  Turns out this is a common problem for some time.  And, an
expensive one repairing it in normal fashion, 100+ in parts and
hundreds of dollars labor.   According to Ford, you must replace the
air handling plenum by 1) discharge the AC and remove the evaporator
housing.  2) lower the steering column and remove the entire dash
assembly 3) remove and replace the plenum; then   reinstall dash and AC
evaporator and recharge system.  I must say this is one of the biggest
fiascos I have encountered in my 40 years of experience owning and
maintaining vehicles.  It's just plain poor design on the blend door
and absolutely no maintainability engineering done to ensure a
reasonable repair process iin the case of failure.  The crafty fix you
ask?

Go to the   Ford truck forum
http://www.ford-trucks.com/forums/forumdisplay.php?f=32 and find the
"free blend door fix" postings for "Jfrank" and "talon dad"  Then find
their photo gallery postings (you will need to register and spend a bit
of time poking around) and you will have detailed instructions with
pictures for how to effect a repair.   It took me less than an hour to
do and cost nothing.  So my system is now working normally.  A side
benefit is by not doing this major dissassembly is you avoid the
possibility of MIF (Maintenace Induced Failure) of other parts.