Rear end whine when towing?

[Cobra99]

I've had my '01 Pathfinder since June. Had 44,000 miles when I picked it up, up to about 55,000 now. Never heard any troubling noises from the rear end at all in that time, and I had towed a few times. Well, just this past Saturday I was towing a boat and the rear end was just whining like crazy. Only towed about 3 miles and never over 45 mph, and as soon as I unhooked the trailer I couldn't hear it at all again. I even thought the noise may have been coming from the trailer, but I hopped in the cargo area and had my brother drive, and the whining was definitely coming from the rear of my truck.

 

I've heard plenty of Mustangs with whining ring and pinion gears from improper setup, but those whine at all times. What would it indicate if mine only whines when pulling a load? By the way, the boat/trailer had to be at most 3000-3500 pounds total, so I wasn't exceeding the reccomendations by any means. I'm thinking I should go ahead and check/change the diff fluid, but I don't want to just mask the issue if it's something that needs to be fixed. I have a warranty with a $100 deductible, but if I bring them the truck without a trailer there's no way they'll hear the noise.

 

Thanks for any of y'alls ideas.

[88pathoffroad]

Check the diff fluid first...it should have been changed at least once by now anyway.