We had trouble with one because my help reversed A bearing holder or something on the left side of the shaft inside the drum. It made to long and impossible to get back together. As I studied the drawing I saw the one part they put in was reversed. Try get a parts drawing. It is what helped us figure out a frustrating problem. It was several years ago and I can not remember exactly what part was reversed on the shaft.
Did you remove the auger tube itself from its end caps or did you pull the whole assembly out complete?
If you haven't done so already I would probably disconnect the header from the adapter, makes the drum 1000% easier to work on and get at. Drop it on the transport wheels, pull the two pins in front, the two vertical rod top bolts and the center link ram pin. Back combine away with adapter attached to combine.