Changing just rear tires for higher load rated tires?

Fish tailing has a lot to do with weight distro in the trailer also. make sure that the tongue weight is close to the recommended and take some weight off the rear to move it a little then see what ya have. The tires as noted by a couple will help as would maybe assist air air bags. there are so many things that can be done to help but not cure all ills. You may need to run the fronts at full pressure to help stiffen them so they will more than likely wear faster but you will have 2 extra right?