500 miles under 50 mph

Not a big deal. The GM I used for towing had the same instruction. I followed it, just took the tow vehicle and trailer on a low traffic secondary highway for the 500 miles. We were going camping at the spot which was a bit more than 500 miles away. By the first day it was broken in for towing.

My view is follow the manufacturer's instructions. I always break in my vehicles according to what the maker says and then some. So did my dad.

I also don't run the vehicle at a constant speed while breaking it in, and back off the throttle every so often to change rpm.

It pays off in the long run.

I would do what they say, and change the rear axle oil after the break-in period.

You don't want to have to buy a new gear set and go through breaking it in all over again, because you were too stupid to break it in the way they tell you...

The break in period is for the gearset and not the engine. I would head the 500 miles without towing recommendation. I would ignore the under 50MPH though once you begin towing...