That happened to me. I "fixed" it once by "helping" the wiper up while the motor is on... this worked successfully only once.
Replacing the motor was not hard. A remanufactured motor is available from Advance Auto Parts for ~$65. Removing the rear hatch panel was not difficult, but be careful not to damage the little snap-in clips. There are 4 at the top and 1 on each side. Also remove the 2 screws holding the strap on. Once they are unsnapped, make sure the glass gate is open and push the panel toward the top of the hatch and it should come off. The rest is pretty self-explanatory.
A brand-new Motorcraft motor seems to go for $120.