Ford Enthusiast

[tx_ag_97]

Vehicle: 2004 Expedition, small V8 (can't remember size, 4.9L maybe).
Problem: When A/C is on it will blow to defrost when RPM +1k, then will switch back to blowing normal when under 1k RPM.
Reason: Leak in my vacuum lines somewhere (or so I've read).

Questions:
#1) Which lines are my vacuum lines? Pics or web links much appreciated.
#2) Where would this leak most likely happen? Joint? middle of hose?

I am by no means a mechanic, but I am comfortable changing all fluids, alternator, brakes, etc, minor stuff. So you'll have to pardon my ignorance as to which lines to examine.

Thanks for any help.