That is your problem. Pull the fill plug and check the oil. I'll bet that it's full of metal. The gears have a break-in period. During that process they deposit a lot of material into the oil. They also heat-up a lot.
I drove my truck for 500 miles lots of short trips, but long enough to get everything up to temp (heat cycles). Then I towed my trailer to work and on some short trips when possible. Once I had another 500 miles of towing I changed the oil in the rear. It was very gray when it came out, not gray from water, but gray from the gears breaking in. Last I checked, I had 40,000 miles and the oil still looked like it did when I put it in. I use Mobil 1.
I would check the fluid level. My 1/2 ton did that, and I had the shop check, and it was 1/2 quart low. They filled it, and it hasn't gotten any worse. I am sure it was low from the factory. The dealers are supposed to check them, but they don't.
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