I didn't understand from your question if you wanted to change key bindings in the keymap you were using in xfce4, or the whole keymap in X. I think this must be done in xorg.conf and not in xfce. This way it will work in X no matter which WM you use.
I just looked in my xorg.conf and I don't have any specification so I must be taking the default. If you backup your xorg.conf and run xorgconfig again it will give you a list of keyboards. Or you could search for xorg.conf keyboard on this forum and maybe there will be a post which has info.
I don't know about your distro but it's normally saved in /etc/X11/xorg.conf