I did not tow for about 1,500 miles on the new truck. When I hooked up for the first time, I held it to no faster than 55, took it easy on starts and acceleration. Our trailer weighs about 7,000#.
I was a Journeyman quite some time ago, and I was always told by the Masters in our shop that axle gears tend to mesh more deeply, the more they are loaded. So by that reasoning, there are parts of the gears that you are polishing that weren't in mesh before. If you climb a long hill, take it easy, let it slow down a bit until you get those miles on the axle. The rear end will be heating up due to the friction.
Hope this helps.
Oh, and we have over 55,000 miles on the truck and no troubles.