Ulysses is pointing you in the right direction if you are having blend
door trouble. Go to the Ford truck forum
and find the
"free blend door fix" postings for "Jfrank" and "talon dad" and you
will find detailed instruction for determining if you have that
problem. If you do 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.
The alternative is to spend 100+ for a new plenum and a weekend tearing
your truck apart to replace it; or paying a shop 500+ to do the job.
Having had this problem myself, 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 belnd door
and absolutely no maintainability engineering done to ensure a
reasonable repair process iin the case of failure.