your problem sounds like a clutch but could be a
tranny prob. does it slip in every gear or only in one or some?
drivetrain warranty is i belive 3 years.
did the dealer confirm what was wrong? if not how can he deny warrany coverage if he does not know for sure what is wrong. if reminding the dealer of this does not work why wouldn't you write a letter or call ford directly? at the end of the day it is their responsibility.
while you are at it, make sure to tell them about how your dealer treated you.