My rear differential makes a noise when I accelerate. It only does it if I have maintained a speed between 30-50 mph and then hit the gas to accelerate, and it only does it in 3rd and 4th gears. The fluid level is full on my differential and I have maintained it pretty meticulously. It isn't a grinding sound, and the truck doesn't lose any power, it is almost just a groaning sound coming from the rear diff. Any suggestions on what it might be?