Having jsut spend the last few weeks becoming very familar with CAN-BUS wiring and shorts there in, I can say this sort of problem can be a PITA, but still need diagnosing and fixing.
First thing, Do a capture of the 2-wires at any single point, so you can see the high and low signals.
To do a volt drop, which I'm not so sure is going to help, you need to ideally find the end points (so a Can topology diagram is needed), and if you have the newest of the Pico range 4x25, you can do this on one channel per wire (as the ground can be floated), the older 4xxxx or 3xxx you'll need to use 2 channels for each wire you test and use a maths channel to calculate the drop.
So using the older scopes, using High wire, but same procedure for low wire, put chaA, onto the Pin at one end of the topology (either the external resistor, or the ECU), then ChB will go to the furtherest end of that CAN-BUS, into the same wire, again at the terminator resistor or ECU depending on how the Bus has been terminated.
Math is either A-B or B-A, either and just ignore the sign, the difference is the volt drop.
Repeat for Can low
Maybe do both at the same time to look and see if volt drop on 1 wire is different drastically from the other? Or coincides with the packets?
Really not sure Volt drop will give you anything, but that's how you can do it.
Best to post back the DTC errors in each ECU, the reported fault from the owner (drivability or just a lighton ), a list of the ECU's on the bus, the topology, what resistances you got measuring the Bus, what that changes by when you remove each ECU in turn (to find individual ECU restances - remember ohms law ... the Parralel version
), do you know what the resistances should be for each? You can measure the 2 pins on the ECU WHEN OFF the harness to check...
Clean all the Pins and retest? Any water ingress? Pin condition?
Lets see some captured data files (and pics for ease), I'm sure we can give you lots of ideas!