I've got a problem on my hands but I'm not sure what is causing it. After I've run at highway speeds (55-65mph) for about 15 minutes I start hearing a bit of a whine coming from what seems like the center to drivers side rear of the truck. I checked the fluid in the differential and added a couple of ounces. That seemed to helped some but it's still there. It sort of comes and goes as well. With tranny (automatic) in neutral, coasting, doesn't affect the pitch whichs changes with speed. Checked axles at hubs after the run, cool to touch. Differential a bit warm but not hot. Any ideas?? Truck has about 65k miles on it. I did have a leaking pinion seal replaced around 30k miles, no leaks since then. Should I replace the differential fluid and while I have the cover off take a look at the gears? If I do that what should I look for on the gears??

Change the differential fluid.