2006 f150 roaring noise

i have a 2006 f150 and when im on the 4 lane driving about 55 mph you cant here any noise,but when i start going up a hill and about the time it starts to lug it will start making a loud roaring noise coming from the rear,but when i kick it down it quits making the roaring noise.it only does it when i hit a steep hill.i put a used rear end in it and you could tell that the people that i purchased it from had the inspection cover off because of the oarnge gasket maker.could they of not set it up right.or maybe a bearing gone bad.thanks