Story has been found here: Spectrum (in 4 parts), by tbrook.  The conclusion is here: Jumping at Shadows.  Thank you for the help, all you commenters!  :D

This is an old specific story I'm looking for.  It starts right where the series starts, just before Jess's death.  Sam passes through his own doorway on which somebody carved a cursed sigil, and he basically loses his mind.  The story is written in a very distinct stream-of-consciousness style from a third person Sam perspective, and I believe this moment is described as "the whole world took a step to the left."

Jess dies the same way she did in the series, and Dean shows up after Sam calls him incoherently from the kitchen floor.  Dean tells him to stay put, because he was already almost there.  The rest of the story is basically an insane Sam-styled warped perspective on the beginning of the series.  Dean is looking for their father, but his search has been completely disrupted because Sam is incoherent.  He can speak, but the things he says are often uninterpretable without any perspective on the things he's hallucinating.  Sam frequently begins screaming for reasons completely incomprehensible to Dean.  Once at a gas station he starts screaming because he is afraid he will fall into the sky.  He keeps seeing a man in a trenchcoat following them.  The television looks like an eye watching him.

I think the story is in 5 parts or so.  I read it on livejournal, and the author had a blue background.  Sadly, that is all I can remember, and keyword searches have gotten me nowhere.  The story is gen, no pairings.  It's implied that the man in the coat is Castiel, but this is never said outright, and I think Bobby shows up at one point, but other than that it's just Dean and Sam.  The style is the most distictive feature.  It read almost like poetry-prose.  And it would be at least 4 years old, probably more.  If anyone can help me find this, I would be so grateful.  It was an amazing story, and I'd love to read it again!