I have a 72 F100 with a bad rear end, I was wondering If the rear end out of my 76 F100 would fit. And how it could be changed?

 

Your 9" rear end from your ' 76 F100 should fit under your ' 72 without further modifications, the only difference might be in gear ratios, which you will find on one of the studs that holds the center section into the housing, the ratio will be stamped into a tag and it will look something like this 3.50.1 or 2.75.1 or what ever ratio you have.