Well, now that I put the search out there of course I find it within a few hours, if not sooner. The story I'm looking for is In my Time of Living by Sevangel at Twisting the Hellmouth. And it wasn't Xander in the story, it Was Dawn, who was actually Mary Winchester's soul come back. After several weeks of dreams of Sam and Dean in danger that leave her almost sleepless, Dawn talks Willow into performing a spell to tell them what is happening. It melds the two personalities together, changing Dawn's appearance so that she looks older and more like John's wife. Using colored contacts, fake piercings, and multicolored hair extensions, Dawn gets a job at Ellen's Roadhouse... Great story; check it out.

Of course. This one is called 'In My Time of Living' and is written by Sevangel. Twisting the Hellmouth is www.tthfanfic.org, search by author. (You can search for stories by fandom and main character, but in this case by author is better.) And while you're there, if you're a Buffy fan, I have to recommend two wonderful BtVS/Law & Order SVU stories. In 'My Daughter...the What?' Buffy finds out that her father is actually New York detective Elliot Stabler. Another, slightly darker version of this is 'The Flipped Side of My Daughter...the What?' where it is Faith, not Buffy who is Elliot's daughter.

These three stories are a fine read and will keep you entertained for quite a while. Enjoy!