Differential whine

in a 90 mustang lx 2.3 auto, at appx. 30 mph you will begin to hear a whine noise. i am unsure at what speed the sound stops if ever. but at 30 the noise "fades in". if you let off the accelerator the noise will "get louder" or just more noticeable. the noise is always under accel and decel. while cruising and maintaining speed the noise disapears, also while cruising if i tap the throttle the noise picks up. while checking my gear oil, i found the level to be less than satisfactory. i put in a bit more until it started spillin out. while i had the wheels off the ground i put the car in gear and accelerated up to 40 mph and there was no noise whatsoever. so i know that it is a load and torque sensitive noise. what im not sure about is wrather or not the fluid level is the root cause of a whine. could there be gear damage? ive had the car for about 5 months and it has had this problem since ive had the car, lord know how long before i bought it. any input would help alot. thanks guys.

Pop off the differential cover and inspect the tooth contact surface of the ring gear and pinion gear. Also check the four small spider gears in the center of the carrier housing. Look for pitting and rough spots. They should be shiny and smooth. If all looks good take out the axles and the carrier housing and inspect the axle bearings and carrier bearings looking for rust, pitting or any other damage. Check the bearing races and the bearing surface of the axle for smoothness.