humming noise from 98 f150
 

I have a 98 f150 4.6l 2wd with 180,000 miles. It is making a humming noise in the rear when I get about 30-40 mph. It was leaking grease from the yoke on the drive shaft, and I replaced and filled it up with oil. This noise is really worrying me. I'm afraid that the diff could be going bad or something, and I have put 180 miles on this truck since I purchased it. Any help is greatly towards fixing this noise is greatly appreciated .