How to switch an output based on specified voltage difference (outside of rail tolerance)?
What you are asking isn't clear, and the schematic isn't clear either.
Regardless, using an instrumentation op-amp in this type of voltage is probably not the best solution.
Instead of detecting voltage levels, you could detect pulses by using a capacitor as a decoupling mean, so you don't need to care about the voltage levels or referencing.



simulate this circuit &ndash; Schematic created using CircuitLab
When data will pass, you will detect the pulses on the line, then have an MCU or some analog circuit to do what you'd like. The voltage divider (use 10k res) will set the threshold of the detection.
C1 has to be small enough to get the edges, this depends on the can speed. You can start with perhaps 1nF. You only need to detect in one of the line.