I don't remember if this was an episode tag to Malleus Maleficarum because of Sam's hurt back or not. All I remember is Sam was driving the Impala, Dean was asleep in the passenger seat and Sam's lower back was really hurting. So Sam pulled off to the side of the road to get out and stretch but he hurt himself further and couldn't move. That's when Dean woke up and made his way out of the Impala and helped Sam back to the Impala. It was all in Sam's POV too and I'd really like ti find this story again. Any help would be appreciated. It was definitely gen and I believe it was on ff.net

Edit: Found! Link in comments.