I will put the link to the video above again so that anyone else can take a look. http://discuss-assets.s3.amazonaws.com/original/3X/8/2/82fda1ce5f7c3bdbdbfe3234ec10c08d647a171a.mp4
Having a further think it is very unlikely that it is a stepper driver if the other motor is working(I am assuming that you have a 1000mm machine though. if you do have a 500mm xcarve then it may still be the case).
So it is likely to be wiring or something is jamming or you have a bad motor. The sort of issue with the motor is small but over a large area it can cause the other side to jam as well over time which would make it look like an issue with both but only ends up as being one motor.
Its one of those hard things to diagnose as there are multiple factors at play.
So you may have a broken stepper motor but it would be best to rule everything else out first. So if the wiring checks out then i would say give inventables a email and see what they have to say. They should be able to sort you out if it is a bad stepper.