I've been doing some investigating and noticed something and I'm not sure why it's happening or if it's related to my problem.
On my L293 (http://focus.ti.com/lit/ds/symlink/l293d.pdf) I have my external power supply running to pin 8 and the arduino 5V running to pin 16. When the Arduino is powered on and the external battery pack isn't, pin 8 is putting out 1.9V ... is that normal?