USB 6002 and differential measurements

[AndyTailored]

I am looking for transient events on DC power supply lines with a USB 6002.  The supplies are independent with separate negative returns.  That means I can't use single-ended measurement method, it would mean connecting the returns together.  So I am measuring in differential mode.  And, it's not working.  

I can use a DMM and measure at the input of the differential AI0 (that's AI0-AI3 terminals), and see 3.3V.  But my program and MAX report somewhere around 0V.  

If I look at each supply independently in single-ended mode, I get the right readings.  

The laptop is not plugged in, so no ground loops.  

I have looked at the diagram below, which I have seen many times but still don't quite understand.  The top left figure says resistors to carry bias currents.  What bias currents would those be?  Something that NI instruments need to operate?  Do I need to include them?  If so, excuse me for being lazy and asking first.