74Mopar-
Your description of the motor working, but the arms only making a half swipe and stopping leads me to believe I may have your solution! if you remove the driver side cowl screen to get to the linkage, you'll see a piece of metal shaped like an "s" which attaches the motor to the linkage. This piece (don't know exactly what it's called) allows the motor to turn the blades in an elliptical motion. I had the same problem you describe. I detached the "s" piece from the motor, and bumped the motor switch until I could reattach it to the motor without bending/stretching anything to make it fit. (the hole on the "s" piece is shaped kind of like an oval and it will only fit when it's lined up properly.) Sounds like your linkage is not moving freely, which will prevent the wiper arms from making a full pass.
This worked for me, and is certainly worth a try before paying anyone to rebuild the motor.
