The most common advice you'll get if you do your own trouble shooting is to swap motors, then cables, then drives, one at a time. The idea is to try to isolate the problem area through a process of elimination. This is your best bet. Even if you have to stock a spare motor and drive, it can be worth it when you're broke down.
You need to take suitable precautions when doing this. The safest method is to remove the motors from the screws they are driving, in case you get a runaway because your encoders are not hooked to the correct axis when doing this kind of swap.