2000 Rear end rebuild

Picked up the truck last night from the dealer and was able to talk to the guy who did the rebuild on the rear end. It appears that I had a bearing go south and throw metal pieces into a few others. Short list included a new left axle, seals, bearings, gears, etc. At 96,000 miles I was interested in what I did to cause a failure and asked him what I could do to make sure it never happened again.
Interestingly, he indicated that I should be changing out the diff fluid about every 20,000 miles. It was his opinion that the synthetic fluid doesn't dissipate heat as well as regular gear oil and gets hotter as well as dirtier. He indicated that whenever he drains the synthetic fluid, it is always pretty black. Thought I would pass this on as regular service intervals recommended by GM goes much longer.