a worn Wiper Motor shaft bushing (#1)(effects both)
a loose Wiper Motor Crank Arm (#7) (effects both)
Worn Wiper Transmission components (effects either one or both)
could be the culprit.
Here's an illustration of the system:
If you feel no slop between both wiper arms, (hold one stationary and try to move the other one), it's most likely either link #12, or crank arm #7. It's rare that the Wiper Moter requires replacement...