Hello Leo,
When using the AI channels in differential measurement mode you need to use the AIGND pins (32,29,27,24,56,59,64, 67). AISense is for use when measuring in the Non - reference single ended measurement mode and the DGND is obviously the ground for digital signals.
The 5volt line can supply up to 1 amp of current and can be used to power what ever peripheral device you require, or can provide voltage/current excitation for a sensor. Obviously you need to use this pin within the spec detailed in the manual.
Without looking at your full circuit diagram I cannot comment to see if the measurements you are reading are correct. Can you verify them using a scope or a DMM at each AI node? It maybe that depending on your circuit connected to each
AI channel that you are saturating the AI channel?
Hope this helps.
Can you post a circuit diagram to help visualise your equipment setup?