Hi John,
This device is limited as you have discovered. Basically you will need to set AIN- to 2.5V and measure the difference and add 2.5V to the measurement.
You could also take a look at our latest device the ADS1220. This part will work more as you expect, but only has 4 single ended inputs. You might find that two devices take up less space and can take data more quickly as it is single cycle settling.
Best regards,
Bob B