I don't know if there is an official post that indicates which episode we will see the return of Ellen, Jo and Rufus. According to IMDB, Ellen & Jo Harvelle and Rufus will return in Good God, Y'all. Here is the link to the episode's IMDB page.
And I stumble across the casting for another new episode of Supernatural, which I think it is called Celebrity Skin (that episode has Paris Hilton in it..) Anyway, who care, but, Stargate Atlantis fans check this out, Paul McGillion (aka Dr Carson Beckett) will be a guest star in this episode. Link.
Check out the Supernatural IMDB site here.