Hi-Welcome to the forum!
Unfortunately there is nothing you can do legally to increase what the manufacturer says you can tow with your truck. If at all possible-I would try to back out of the purchase of the trailer. You have purchased too much trailer for your truck. There is a big problem with RV dealers misleading those looking to buy trailers. Once a customer likes a certain floorplan-the salesman more often than not tells the potential buyers that their rig can tow it, when in reality they cannot.
If you decide that you want to change the rear end ratio-in the event of an "at fault" accident what you were towing and the capacity from the factory will be closely looked at. Don't mean to scare you but that's reality.
