Old story but the shape shifter they killed in Nightshifter had sister's and the group of them hunt Sam and Dean. They have the ability to get into Sam's head when he dreams almost drives him to suicide. There was a scene in a camping bathroom where he thinks he is dreaming but he almost shots himself in the head. I think it ends up being wincest but the story center's on Sam and Dean being hunted across the country by people that can change their shape at will. it's driving me crazy try to remember who wrote this.

EDIT - found in comments you guys are awesome!