Thanks Simon,

Managed to get a wiring diagram for this vehicle and it seems that the network (GMLAN) for the ECU involve only the ABS, ECU, and BCM, must have had a bad connection on the ABS when reconnected the physical layer returned to normal i.e. 60 ohms, CAN signal voltage dropped to a more respectable voltage level and CAN positive looks ok; but CAN negative is still corrupted. Checked connections feeds and grounds so going to get ECU checked first as a HT IGN fault caused this problem, but I suspect the ABS as well as the signal doesn't improve with disconnection.

Mark