Had another look at this today, scoped the sensor, perfect consistent trace. close look at reluctor & checked crank end float OK, checked wires back to ecu OK.
These are prone to ecu faults, (engine mounted, vibration I guess,) so I think looking like possible ECU fault. Not causing any running issues at the moment, so will have to see if the owner wants to take it further.
Anyone found contact for test & repair truck ECU's?