T - Bone Junction
This one requires "The Secret Armory of General Knoxx" DLC.

This one is very easy to do,and will bring you a fine amount of cash (and XP of course) in the beginning of the game.
When you reach the mission in which you need to fix the broken med vendor (you don't need to complete it),you can then use the fast travel station to travel to DLC locations. Travel to T - Bone Junction,and after the intro scene will finish you will get a mission to simply meet scooter. Go to his garage,then turn in the mission for some easy xp. After that,you can do a mission where you collect monster truck parts. In the mission,you need to be very careful not to get hit by the asassins you encounter,as they are very high leveled and can kill you almost instantly. Try to avoid them,and focus on collecting the monster parts. When you are done,reutrn to Scooter and turn in the mission. Then you can do another mission,in which you simply build the monster truck. Turn it in and you are done with that.

Doing this will get you to level 6 - 8 very early on,which I found out to be pretty handy.

The Lost Cave

This is a side - quest you can do by going to the Fyrestone bounty board. The quest is called "Shock Crystal Harvest", and it is a level 15 mission,however you can do this quest at level 12 minimum.
The big XP givers here are the Larva Crab Worms (1000 - 700 XP or so),which are very easy to kill and give you quite alot of XP. If you farm them long enough,you will reach level 17 - 18,depends on how much is enough for you.
In addition,there are many red chests in the map,2 of them are in the end of the map where you will find alot of Alpha Skags,and sometimes badasses. These enemies also give you alot of XP,and sometimes the badasses will drop an Eridian Artifact,which are a must have for every player.

Dhal Headlands

After you leave Fyrestone,you will meet a person named Ernest Whitting. He gives you a main story quest,and a side quest for Skagzilla. Skagzilla is a boss,and as you may expect,gives you a handy amount of XP (if you manage to kill him,that is). A good way to avoid his attacks is by running and jumping,also make sure to shoot his mouth when he roars or does his beam attack,it can take down alot of his health in no time.

After you rescue Lucky and did the mission where you power up the fast travel station,there are many sidequests on Lucky's bounty board. I personally,do ALL of them,and I suggest you do aswell. One of the quests involves an eridian artifact in a cave. There are guardions in this cave,which give you an insane amount of XP each (I will get to those guys later on).
Once you killed the guardions and turned in the mission,you will recieve a level 2 explosive eridian artifact (Very overpowered with Mordecai).
Then again,you can farm the Guardions or Skagzilla for a little more XP and then move on (at this point you should be overleveled,so they won't give as much XP as they gave you before).

The Trash Coast

The Rakk Hive is one of the enemies that gives out the most XP in the game (100,000 XP if close to your level). He is very easy to kill if you just go behind the rocks where you entered the area,his stomp attack will not get you.
I don't have much to say about this one,he has alot of health but he is very easy to kill,just aim his eyes and shoot them, easy as that. Dodging the Rakks that he sends out at you isn't too hard aswell,and the Rakks have very low health too.

The Descent

You will encounter a Badass Guardion when you first enter The Descent. Guardions have a very high shield capacity but they have very VERY low health. I suggest using shock weapons against them, SMGs and Shotguns are the best. After you get rid of the Badass Guardions and the 2 other flying ones,there are 2 red chests and a white chest in the area. I find The Descent to be more of a loot farm rather than an XP farm.

Eridian Promontory

This is my overall favourite method to farm XP in the game,it is also a very effective way to reach level 69 in playthrough 2.5.
There isn't much to explain in this one,simply run to the start of the map to the entrance of the vault and kill every guardion you see. By doing this,you will level up once or even twice every run. There are also several loot chests in the map,so go check them out and hope for a legendary!

Guardions can give you from 7,000 to 20,000 XP

This is my second guide,I apologize if the format isn't the best,but I just wanna help out newer players in the game. Feedback is always appreciated,and I would love to hear more ways to get fast XP,and if I like them I will add them to the guide.

Thanks for reading!