I have to confirm that its the diff and the fluid will take care of it. I got mine changed not long ago and for awhile it was not making a difference but actually just today, after probably about 75 miles did the diff FINALLY quiet down nicely. I think the parts need to get completely lubricated with the new stuff which from my understanding is an additive that is a bit "thicker" and helps with providing more "cushioning" for lack of a better term, between parts.
So if you get the fluid changed, make sure you give it a chance to work! I thought mine did not fix my problem. Amazing the sounds a differential can make! Sounded like a bag of rocks sliding around back there