I am being told I need to have my rear differential replaced, $3500. Is this a fair price?

What is your question?

I have had all my scheduled maintenance, oil changes etc.. faithfully. About 6 months ago I was hearing a humming coming from the driver's side rear. Thought maybe it was my brake pad and disc but, when I went to get my oil done last time they said it was actually my rear diff and it was recommended to have a new rear diff replaced costing, $3500. About 8-10 months ago they replaced my diff gaskets due to a slow leak (their observation) so they did that when I had my 95K tune up. Anyhoo... I just want to go into the dealer with some knowledge cause I don't want to be slighted. I hate to say it but, I am a woman and I do not know ALOT about cars but, I do know and have experience with getting ripped off. :)

What makes this problem better or worse?

Letting off the gas pedal seems to curb the noise. Noise has not increased but, still there

How long has your 2018 Cadillac ATS had this problem?

6 months