The AD1115 has a buildin reference. No external one needed (and not possible to add).
All you have to do is drop the 36volt with a voltage divider to one of the ranges of the PGA.
A convenient one is 4.096volt (PGA gain 1x).
Not sure why you need differential mode to measure a single voltage.
Note that voltages (any mode) have to fall within supply and ground limits of the chip.
Leo..