I have what I think is a rear differential that is failing. I have a fairly loud whine that sounds as if it is coming from the rear of the car - speed dependent but not gear dependent so I think I've ruled out the transmission. I've looked for leaks and found none. All the joints seem to be in good shape so I assume the whining is coming from inside the differential. What parts would be most likely to fail? And is it common to replace parts or the entire differential?