This one has been driving me bonkers and three days of searching has turned up squat.
It happens, I believe, sometime after Sam gets his soul back from hell and before Cas goes Godstiel. In lieu of that, the boys (Dean, Sam and Castiel) apparently find a piece of God's grace that Castiel had been planning to use as fuel in the war.
It begins with the boys fighting with another angel (Raphael I think) trying to steal the grace and Dean swallows it.
I don't remember if Bobby was there but it had either Balthasar or Gabriel, not sure which.
They all worry a lot since Dean is not supposed to be able to do that and he will eventually explode. Cas tryies to use his own grace (that he used originally to piece Dean together when bringin him back from hell) to keep the God grace in check.
Dean goes all righteous and goes to Heaven to act as bomb but has a heartfelt scene before where he says goodbye to (paralized and protesting) Sam and Castiel, where he gives Sam his jacket and Cas his ring. It's somewhat Dean/Cas, but nothing has happened between them and nothing did in the fic, unless he gave Cas a goodbye kiss, not sure there.
Dean goes exploedy and meets God/Chuck that tells him he´s going to ground Castiel for putting Dean higher than God, but that he's also proud of him and then he returns Dean unharmed to earth.
Anyone know this one?
Edit:
FOUND!
It's :
Are You There, Dean Winchester? It's Me... God