It's more likely the drive than the stepper. Easiest way to tell is to swap the drives with a different axis, and see if the problem moves to the other axis. If not, it's the motor. If the other motor can be turned by hand, it's the drive.
If it is the motor, and you can figure out the voltage and current ratings, you should be able to find a similar motor for much cheaper. Looks like it's a 600 oz-in motor, but I couldn't find the exact specs.