dhenry:
Following is my project brief:
You should try to use the mcu to control the "current sink driver", not supplying current to the current sink driver through the load.
The UDN2981A is a current source driver, so it doesn't meet the brief. The ULN2803A is a current sink driver, and will do this job if you can tolerate the voltage drop of that device. Otherwise, use 8 medium-current NPN transistors such as BC337, or 8 medium-power mosfets.