Rear end noise

I recently bought a IV but I failed to notice the loud noise it was making in the rear end. Whenever I start to speed up or get to 3rd gear there's a loud vibrating noise. The noise starts to go away when I'm slowing down as well. I've brought it to two different shops and they didn't find anything wrong with it but the noise is very annoying. Especially for the people who sit in the back. The car is lowered but I don't think that is the problem. I was wondering if it may be the diff that may be the problem. Also, my AYC light won't turn off. Any ideas about that?

Thanks!