Thank you dreamer, I have replaced the U-Joints not too long ago, so I will definently look into the differential; plus the fluid is in need of some care, so I will change that as well.
Engine Oil lifespan has definently improved over the years, but if you drive your car in the city, that is considered heavy duty driving. I live in a very weather damaging city as well so I change mine every 3 to 3 1/2 months.