I am not an expert but have experienced similar problems.
In my situation I had "heat stuck on" and "air comes out of defroster vents no matter what mode is selected." These are 2 different problems.
I fixed the "heat stuck on" problem by fixing my "blend door actuator" as described on this webpage: http://www.blenddoorfix.netfirms.com/
To the best of my knowledge the "defroster vent only" problem is due to a vacuum problem. I haven't yet resolved this...it was never a big deal to me but recently I've been wanting to fix it just for the sake of fixing it.
98 Ranger XL 2wd 2.5L 5spd 3.45 gears
15x7 Pacer black steel wheels, 225/70r15 Cooper Discoverer ATR tires, LED front side marker lights & license plate lights, red LED dome light
GoLight #5149-M (work/off road use only).
Suspension lift, 31 or 32" tires, 4.something gears