I have a whining sound that intermittently has a grinding sound with it (when decelerating). Around 55 MPH is when it is the worst. I am guessing it is the diff.
I have already replaced the drive shaft with one that can be lubed.
What does it sound like when a differntial is on its way out?

 

I've had a center diff fail on a vehicle before, and the sound is much as you describe, grinding clunking and eventual lockup. My center diff locked the wheels a few times on gravel and literally broke loose and kept going. You'll be able to tell which diff is bad when you drain the fluid and find lots of metal chunks.