GoForSmoke:
The 2560 can treat one analog pin as ground and measure another relative to that?
I would not use the word "ground". Ground has a special meaning related to the Atmega's power supply.
In my earlier post I just said that it can measure the voltage difference between two analog pins.
If you don't believe me read the datasheet and try it. Connect a low voltage battery between the appropriate analog pins without making ANY connection between that battery and the Arduino GND. I learned about it from this Youtube video and I have tried it.
...R