1998 Taurus air won't blow out front vents

When speeding up to get on the freeway with the air conditioner on, I felt the engine drag and the air shifted from the front vents to the defrosting vents and no longer would blow cold air. I replaced the air conditioning diod and now have cold air. However, the air no longer blows from the font vents when using the air conditioner or vent option on the heating/cooling switch. What could the problem be and how difficult would this be to fix?

Look for a vacuum leak in the system. If there is no vacuum leak, suspect the switch.