Why won't 2002 Ranger blow air out dash or floorboard vents only out defrost vent?

Most likely cause is that one or more of the motors controling the blend air doors have failed. Another possible problem is something fell into the defrost vent and is blocking the door from closing. If air is only coming out of the defrost vent for all vent positions, then you likely have a vacuum line that is broken or the vacuum reservoir (3" ball mounted near right front of engine compartment) is cracked or leaking. Try removing the lines from the reservoir and connecting them directly (to check the reservoir integrity).