before you drop that kind of cash, i think you would be able to do each window for about $10 and 45 minutes. ive had 3 fords that did this, and its a common problem. if you hit the switch, and hear the motor running, but the window is not moving - its the bushings in the motor. there are 3 plastic parts that break down inside the motor. all you need to do is yank the motor, take it apart, and replace the 3 plastic bushings inside. you can buy the bushings at autozone or pep boys for about $10. they are with the "HELP" brand OEM replacement parts. (like the door strikers, knobs, etc.) good luck.
here is a good write up. its for the SN95 body, and not the fox. however - its exactly the same procedure, and the motors are the same.
How to install on window lift bushings. - Mustang Forums at StangNet