[Mark McLeod]

To eliminate software as a possibility, in LabVIEW, the start page has a link in the lower right to FRC Examples. Go there, to the Robot & Motors examples to test a generic Arcade Drive.
Make sure to change the project IP to your own and just use the "Run" button. Don't bother to download it.

Another test is to move just the PWM end plugged into the Jag to the working side. Leave the Digital Sidecar end alone. That'll tell you if your code is working correctly.

Electrical:
You swapped out for a different Jag. That's probably the easiest way to check.

Mechanical:
Is the drive train easy to turn by hand on the side that's giving you trouble?
The gearbox could be giving too much resistance.