When you get the diff making funny noises it's time for a rebuild with some fresh grease. Over time the grease breaks down or if you get water in there it happens a lot faster.
Assuming you have ceramic diff balls usually all you'll need to do is a fresh set of diff plates and clean/re-grease the balls and plates. If you're on the standard carbide diff balls rebuild it with new plates and see how smooth it feels in your hands, it it's still very lumpy you need a new set of diff balls. Don't run it with a worn set of balls or you'll just ruin the plates straight away.
Remember the diff plates have one (shiny) side which is what's meant to run on the balls, it's not a good idea to flip them and use the other side without doing some work on the 'dull' side - much easier to just put new plates in
