I was a level 51, stuck grinding through the quests and killing every enemy I could find when I stumbled upon this method of farming.
On your second playthrough, go to the Knoxx Armory DLC. Play through a few of the beginning quests until you get the Motor Head quest. Start the quest, gather the info, and then go to where Motor Head spawns.
Kill Motor Head (I use a 4x Fire SMG and it whittles him down in just two or three clips - I also drop my turret). You can continue to back up the ramp outside of his garage and he won't follow you very far. This allows you to shoot him from a distance without getting killed.
After you kill him (and you gain massive experience) simply press "start" on your controller. Exit. Then load up your character on Playthrough 2, drive back to the Motor Head spawn, and repeat. It eventually takes around 45-90 seconds per kill / save / reload. As a level 51, with a +24% experience equipped, I've been getting around 100,000 experience per kill.
This is by far the easiest way that I have found to farm experience once you're at a higher level.
Alternatively, after killing Motor Head, you can open the trash can in his garage which spawns one of the Loot Midgets. This midget, in my opinion, is harder to kill than Motor Head and only gives you 1/3-1/2 of the experience, so it might not be worth it.
Also, even after you've leveled up completely you can use this method to raise your weapon proficiency levels.