You will need to check each and every bush, steering or suspension joint for play - they will need to be absolutely perfect as any play will be multipled due to the lift.
Also checking all mounting points for steering components etc.
The CAN bus is differentially driven, that means whenever the CAN+ line is high the CAN- line is low and vice-versa. So the average voltage between them is always going to approach 0V. Your 0.3V is close enough to 0V and is within the normal range for what would be expected.
A voltmeter, therefore, is not the correct instrument to test a CAN bus system.