Thank you for the ideas Mike,

Last night I verified the tachometer output is getting to the driver inputs okay. When I was turning the motor by hand, I would see ~+- 0 to .3 volts.
Question, should the tachometer shield be connected at the motor, or the driver, or both?

I next verified the drivers are using differentiated input.

Will I have to change the drivers input mode to use your idea of a variable voltage input with a battery and potentiometer?

Thanks,
Bryan