You've been talking to off-roading people, haven't you...?

Most of the break in stuff I'm familiar with is for trucks with low gears and big tires, etc. that make a lot of heat from big loads (from big tires, low gearing torque, etc). The heat cycling isn't for the gears, it's to allow the oil to cool down. If the oil gets too hot before the gears mate and work harden, the oil will break down and the gears will burn. Once the gears mate and work harden, the oil won't get as hot and this isn't a problem. For cars you can usually drive normally (sanely) for a few hundred miles to keep the loads moderate during break in. The loads aren't as high, the heat generated isn't as much. I've heard that 15-50 miles is usually the rule of thumb for first time drive and cool down, but this is a big debate. This is another case of applications being different, so what is necessary for one isn't needed for something else. This has to do with how big your gears are, how much power, etc., also. A Ford 9 inch in a KA 240 won't get very hot, but a 1985 SUbaru diff in a Mustang would... Just off the wall examples to make a point, everyone simmer down.

Basically you're trying to get everything to work harden before putting full load on it. Then it's strong enough, and you're fine. If the gears aren't highly loaded, the temp is fine and fluid breakdown isn't a problem.