Suburban hum

On my 98, 4wd, 1/2 ton, I have a low pitch, oscilating, hum coming from the rear that is especially noticeable when I hit 65-72.

I had a similar problem and vibration before that and had one of the u-joints replaced which fixed the vibration, but there is still an annoying hum, with maybe a slight vibration. I have also had the following done since then:

1) New axel bearings 2) New carrier bearings 3) New tires 4) New ball joints

The mechanic wants to have the balance of the drive shaft checked next. He said he sends it to a place that spins them.

I'm just afraid that I'm getting into a time comsuming, expensive troubleshooting process at this point, and could use any advice or confirmation of the approach.

I took it on the highway and put it into neutral while it was making the sound and the sound didn't go away. I don't know if that eliminates anything. In my mind, it does eliminate the exhaust as it almost sounded like it could be causing the problem.

Thanks, Fred