2005 Ford Mustang Differential squeak

Lately I've noticed my car making a squeak squeak squeak sound. It is easier to hear at low speed and picks up with speed. I changed the brake pads and rotors but it didn't fix it. I jacked up the car and rotated the tires and listened. The sound was coming from the middle of the car near the rear end. I disconnected the driveshaft and the noise remained. I put wd40 on the pinion flange where it connects to the differential and the sound went away. I decieded to grease it real good. I took it for a drive and the sound came back. What could cause this? I've isolated it to the differential but wouldn't it mean it was a problem on the outside of the differential it spraying it temporarily fixed it?