Truck Repair: rear axle noise, castrol synthetic, diff gears

Question
I have a 2004 silverado 2500 4x4 with 6 litre and auto trans. There is a very high pitch noise coming from the back end of the truck. The dealer checked it and replaced the diff. gears and bearings and the noise is still there but a little quieter. It is on accel. only and at 80 and 100 Kph. If you accelerate hard through these speeds you cannot hear the noise, just on light acceleration or when pulling a load. It is a much higher pitch that the typical rear end howl or growl. I changed the oil after 5000 km and the oil was black. No metal just very dirty. I replaced the oil with castrol synthetic and added 500 ml of XR-5 oil stabalizer. The noise is still there but hardly noticeable. 1 Any ideas? 2 Is it ok to use xr-5 with a limited slip diff? 3 Is there damage being done to the gears?
Thanks
George

Answer
Hi George,
In my experience, it is uncommon for the noise to go away under heavy acceleration if it is a gear mismatch. Actually, it usually gets worse, and only subsides during coasting.
I am wondering if it might be tire tread noise.
A quick swap with a different vehicle would help.
Another thing quick and easy would be to over or underinflate the tires and go for a ride to see if that at least had some effect on the noise.

Did the dealer replace the Pinion gears when replacing the diff gears? It rotates about 3 to 4 times as fast as the axles, and would be a higher pitch.
Van