This is a baffling one then
Like you say you have no trouble with your 2wd but then the system is as you know is different.
Just to check though. Are you still on standard hoses and also have you trialed it with out the thermostat in the housing??
One other thing is check for splits, this may draw in air maybe??? Sorry if I am waffling on a bit with all these ideas

Put a picture off your engine bay on here if your not sure if you have connected something up wrong??