I am sure back then they still used Kings, Jacks, and Headers. Since its around the door and most cracks occurr here and above windows, you migh have had the header slip due to shifting of the Jack. All this MIGHt require is to remove the trim, remove the door frame and see the rough opening. Put a level on the header and see its bubble. If this is the case, raise the header and install a new jack and secure it carefully, then reinstall the frame.
Regardless it will take opening up the door to see the rough opening.