I decided to build a small board that lowers the voltage and adapts the connector types. It was a little tricky finding a efficient voltage regulator that was small and powerful enough. The people on the Trossen Robotics Forums helped though. Finally settled on the SR05S7V2.
Wiring was straight forward:
AX-12 Data -> XL-320 Data
AX-12 GND -> XL-320 GND -> SR GND
AX-12 VDD -> SR V-IN
XL-320 VDD -> SR V-OUT