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 .

 

with 180k on the ticker...it certainly could be that the rear end is in need of some attention....bearings, seals etc.
other things to consider;
It couldn't hurt to have the drive shaft and U joints checked as well...maybe rebalance the tires too. the tires themselves maybe maybe putting up a noise at the stated range??
it maybe that the rear tranny mount is not what it should be either.

Is the tranny filled to the proper level and correct fluid

 

The tranny has the correct amount of fluid and there isn't any sign of leakage. I know it isn't the tranny because I can hear it. I'm wondering if it isn't a bearing going bad.