I can't help with the door panel issue, but reading the first post on this issue , Bob seems to indicates that all 2003 and 2008 XJ8L were aluminum bodied. I thought the first all aluminum body XJ8L was the 2005 on up to the present. Being that I own one, and knowing it is all aluminum.
A quick test for your car up to 05, is with a small pocket magnet.
Chuck
05/XJ8L