For the sake of the narrative, here's what I've done to make this an AU. This is set post-apocalypse, but everything went done a smidge differently.
1. Sam said yes to Lucifer and jumped in the pit, then woke up the next morning in the same field, feeling perfectly fine. He then went looking for Dean, who did the whole "are-you-real-or-something-evil-trying-to-kill-me" routine, and the two settled back into a pre-apocalypse routine of hunting scary crap.
2. Cas saved Sam for Dean, but never actually told either of them that. He brought back Sam with his soul, and the whole soulless!Sam and Cas working with Crowley thing didn't happen in this AU. Sam does remember hell, and his time there felt like about a third of a year before Cas got him back. He has nightmares, but nothing worse.
3. Gabriel did die when he faced Lucifer, but God brought him back and sent him back into hiding, but with a new mission to keep an eye on both Cas and the Winchesters.
I think that's more or less it, but trust me I'll update this if I think of other things that are different. Mainly, Sam's got a soul, Cas isn't a traitorous jerk, and Gabriel's alive.
Sweet. Read on.
Chapter 1 - Nightmares
The crappy motel on the side of the road in the middle of nowhere, Indiana may not have been a five star establishment, but it was good enough for two of earth's greatest heroes. The case that had brought the Winchester brothers to this particular dirt speck on the map was a smattering of demonic activity, but it hadn't been a big deal and they had cleaned house in less than a day.
Sliding the key card through the lock, Sam Winchester flung open the door to the room with little ceremony and plopped down on the closest bed. "Well that was exciting," he said sarcastically.
His older but shorter brother Dean rolled his eyes, tossing a dufflebag onto the bed Sam had not claimed. "They can't all be the apocalypse, Sam," he halfheartedly teased. Joking about the end of the world wasn't something they were really able to do well.
This time it was Sam's turn to roll his eyes, which he did before laying back on the bed. "I'm really tired for some reason," he commented, as if discussing the weather.
"That's kind of what the bed's for," Dean replied, amused. "Why don't you take off the jacket first though?" He gestured towards the coat Sam was wearing, which happened to be encrusted in demon blood.
"What? Aw, man! Gross! I like this jacket!" Sam stood up and continued muttering on his way to the bathroom, scratching at the stains as though trying to coax them out of the fabric.
Chuckling, Dean took the opportunity to change into clothes more comfortable for sleeping, then tucked himself into his twin bed.
Sam came out of the bathroom a moment later, still grumpy about the jacket, and changed into his own sleeping attire before climbing into the bed he had claimed. "Night," he called lazily.
"Mmph," was the only noise Dean could reply with as he drifted to sleep.
At four in the morning, two screams of pure terror ripped from the mouths of the sleeping brothers.
Sam and Dean sat bolt upright in bed at almost the exact same time, torn from their respective nightmares by the sound of fear from the other.
"Hell?" Sam asked his brother, almost afraid of the answer.
Dean nodded, resigned. "You?"
"Same." Sam threw off the covers and stood up, shaking his head as if to rid it of the terrible images. All he managed to do was get hair in his face.
Dean was distracted from his own nightmare by sympathy for his brother. Sam was plenty used to being woken up by Dean's fear of the pit, but Dean was still getting used to the fact that Sam was dreaming of it now too. His younger brother had admitted once that he had counted the days of torture at Lucifer's hands inside the cage. It had been exactly 130 days before he had woken up intact in the field where he had jumped into the cage in the first place. At the time, Dean could only think of the nearly fifteen thousand days he'd been in the pit, but he never even brought up the massive differences in their time below. For one thing, hell was not the kind of subject for the comparison game. For another, Dean knew that Sam's time in hell had been a very special brand of torture. From the look on Sam's face right now, and every other time he had woken up screaming, Dean could tell that there was no one more mastered in the art of torture than Lucifer.
The amount of time Dean had been topside since his own trip below had given him enough distance to recover from his nightmare with relative speed. He gave up on sleeping anymore that night though and got out of bed to join his brother. "Might as well watch the sunrise," he muttered sarcastically at Sam's inquisitive glance.
That made Sam's lips twitch in amusement, but the ghost of the devil still lingered in his eyes.
"Hey, why don't you get online?" Dean suggested. "See if there's anything else to take care of in this area."
Sam grasped onto the suggestion like a life preserver, agreeing quickly. "Sure, never hurts to be thorough," he replied, already distracted by the glow of his laptop. "Why don't you go get coffee?"
Dean gave him a withering look. "Dude, it's 4am. Nothing's open."
"Uh-huh." The disconnected expression on Sam's face said that he wasn't paying attention.
Sighing, Dean pulled on his jeans and grabbed a jacket to throw over his t-shirt, walking out the door with the keys to his most prized possession in hand.