My 2000 Excursion started having a/c problems today. It doesn't matter what position I put the switch in, it only blows out the defrost vents. I put it in the floor position it blows out the defrost. I put it in the a/c position it blows out the defrost vents. I suspect a vacuum hose, but has anyone had this problem before.

 

I don't have any experience with the Excursion specifically, but that does sound like a vacuum problem.

 

There should be a vacuum accumulator under the hood somewhere that supplies vacuum for moving those doors. Either that accumulator or the vacuum line to it may be leaking. I'd check that first.