Edit: FOUND! This fic is by
trolllogicfics and can be found here. Thank you so much!The fic is called "One Shot" and was posted on LJ quite a few years ago. I cannot recall the author's name and Googling such a common phrase has left me stranded. It's very short, probably around 1k, definitely not more than 2-3k.
The fic is an AU of the end of S1 where the boys died in the car wreck, and were somehow sent back in time. It's from Mary's point of view, from before the fire. She gets a car from Bobby Singer, whom she notes looks oddly old, and eventually discovers this car is haunted by two older boys who died in a car wreck. (At one point the trunk opens to reveal an arsenal of hunting supplies Mary didn't put there. The older ghost mouths, "My car.") The ghosts won't appear around John or talk to him, but they aren't malicious, so Mary is happy enough to have them along. Then young Sam and Dean start saying and doing strange things, like asking for salt in the grocery store, and one night Mary wakes (I think, this part's fuzzy) to find Dean at her bedside, telling her someone is in Sam's nursery. She realizes that the ghosts are her sons in the future, takes the Colt, and shoots the yellow-eyed demon.
It's been awhile for me so some of these details might be fuzzy, but it was one of my favorite fics of all time and I am heartbroken I can't find it. I know I commented on it, but I don't know if there's a way to look up your comment history on LJ, and, of course, my delicious account was deleted quite some time ago. Any help would be very much appreciated!
(I hope it's okay to post here even though I'm no longer active on LJ. This is the only place I could think of to go!)