The very first thing I would be doing is check the half shafts also.If you suspect one but aren't sure.Pull back the dust boot and look for discolored metal or dried out grease.If you still suspect the diff,check the oil level.If it's low you could be having bearing problems.If you'r still not sure,drain the gear oil and check the oil for any signs of metal.If the half shafts and diff check out ok,then you need to start checking out you'r wheel bearings.I'd hate to see you wasteing money by changing the wrong parts.