I am going to say if it keeps needing the diff fluid changed then there is a possibility the differential is on it way out. I bought my car used with 11,000 miles on it and every time I changed the fluid there was more and more metal particles in it and black as tar
I beat on it a few times at the drag strip so it was my fault also that it eventually failed. One day it popped making a right hand turn and from that point on when you let off the throttle the gear whining was horrendous:( Don't want to freak you out but that is what happened to me.