Around New England we use stiffnecked fairly often. It is often connected with a superior attitude or snob-iness.
"Mrs Potter is very stiffnecked about her position in society."
Stubborn is used more in connection with someone who will not yield a point or perform an action.
"Five-year-olds can be very stubborn when asked to share toys."
"Even faced with proof, he was too stubborn to admit the point."
I do not consider them interchangeable, or use them interchangeably.