I've played through Playthrough 1, went and killed the Leviathan in the Pirates DLC, played a bit of Playthrough 2 (1/4th of the way), then I went and beat the Pirates DLC on that playthrough. I was lvl 45, so the loot at the end of the DLC was of that lvl. After playing some more in normal Playthrough 2 (up to lvl 50), I decided I wanted better loot, so I went to farm the Leviathan chests again, but it is still lvl 45. How do I get it to scale up to me at lvl 50?
As I note in this related answer, a mission, rewards and mission enemies scale to the level when the mission was taken. If you take a scaled mission at level 45 and level to level 50, unfortunately the mission and its rewards will still be level 45. This is why it's important to wait until you're maxed out in Playthrough 2 before taking on many sidequests.