GND difference between CAN nodes?
Are you sure the message errors are a result of the CAN bus and not a result of the performance of the rest of the node(the PIC circuits)?

CAN is differential, as long as you don't exceed the common mode voltage limits of the transceiver the relative ground levels shouldn't matter and there is no need to send a ground connection with the CAN signals (maybe a shield but thats a different story).

However with isolated power supplies yet the ground connection being there you well could have some ground current loops that are causing problems with the rest of the circuit.