Hi all - I need your ideas on what to check. I have a 2005 F150 9.75 3.73LS rear end diff. I notice a whine when I accelerate between 40-60 miles per hour. I also noticed some residue (oil) around both rear axles at the wheels when I was changing the brakes. I am getting ready for a long trip (1500 miles). I plan to replace the axle seals and bearings. Could this be causing the whine or is it more likely to be the differential?

I will be changing the diff fluid tonight so what can I look for / test to see if the diff is the problem or the axle bearings? The truck has 214K miles on it.