Just a thought: I have noticed that on some XF's, that the sagging headliner MAY indicate a water leak. Especially if the headline problem is in the back hatch area. XF's have had leaking problems under the wing on both the NA & SRT. I would check in the area where the emergency flat tire equipment is in the back. If you have water there, you have a problem that you need to get to quickly or it will cause bigger problems. Just speaking from experience. Good luck!