Thanks for the reply, Baddo. I know that locality wasn't the issue because when I tested the script in MP, I was the only player running a non-dedicated server, so therefore the script could only be run locally.
Okay, I'll go see if a bug report exists for this. Thanks for your suggestions.
Edit: I tried it again lasta night. This time, I put in a line that checked specifically if the unit was local to the machine. The game didn't crash that time, and the disableAI command worked just fine. How odd. Maybe later, if I'm not feeling lazy, I'll try removing the local check and see if it crashes once again. I don't see how the local check could have made a difference if my original MP test was done with only 1 player (me) that was also acting as the server.