when im driving down the highway at 60-70mph i hear a loud humming noise that i havent heard before. is my diff going out.

 

yes, don't drive your truck...


no seriously, that could be coming from SOOO many places. could be the diff, but it could also be front or rear wheel bearings, u-joints, transmission, t-case, clutch, and so on and so forth...

another thing to consider, it's hard to tell by your avatar, but what kinda of rubber are you riding on? could just be the tires; from what I've experienced, most get louder as they get older, especially if they wear uneven

 

Only 60-70, not at 55 or 75?

In my experience, diff noise is usually about the same volume regardless of speed - just the pitch changes.