play the 2nd playthrough but just do the missions related to advancing the main story.... you're basically going to do the 2nd run just for the sake of finishing it. When you finish the 2nd time and then go back to playthrough 2, you will still have all the side quests to do, but all the enemies will level up to the max and the weapon drops and chests will all be level 48 or 49. That's not to say there won't be crappy weapon drops, they'll just be level 48 crappy weapon drops.
at that point i find that New Haven/Crimson Fastness runs give the most bang for the buck. There are 5 chests in New Haven (2 red and 3 silver) and 5 red chests in the Crimson Fastness. I've farmed plenty of good weapons there.