I bought my '05 Chevy dually new from a dealer 4 hours away. I got in it and drove it home on interstate at 75 MPH. I was going to the lake the next day and could have taken the old truck. But I decided I just got a new one, I'm going to use it. So, I hooked up the 4,500 pound boat and towed it 1.5 hours to the lake and 1.5 hours back. Then I finally got around to reading the manual that says not to tow in the first 500 miles and not to exceed 55 mph. Whoops! I now have 208,000 miles on the truck and it's been the most reliable vehicle I've ever had. Absolutely no rear end problems at all.
I do recommend following the manual but in my experience I didn't do any harm.