My mechanic is telling that my ford expedition 2006 4wD eddie bauer, that I have to replace the differential rear. I just change the front cv axle on the both sides but I still have the weird noise. Please help me!!!

Hi, I have a mechanic check out my SUV. He saying is the transfer case but I'm not so sure about that. I spend so much money in this expedition 2006 4wD because different mechanic's Keep telling me that something is wrong and when I fixed I still have the same noise. But one thing is truth, the noise is coming from the transfer case and I just change the oil and still the same. Please help me