How often you change your diff fluid is dependent on how you drive your car, level of modifications, and level of comfort. If you are tracking your car you need to change it much more often than if it is a daily driver.

IMO there is no such thing as a lifetime fluid. Assuming normal usage I would change both the Transmission fluid and Diff fluid every 50k-60k miles. A number of the MB guys have had issues with their transmissions at a 100k+ miles after not changing the fluid.

Think of the fluids as the life blood of your car. 3, 4, 5 transmission fluid changes are a lot cheaper than replacing the transmission.

I am being a bit excessive in my diff fluid changes but I have done it every ~20k miles. The qualifier being that I had a Phantom Grip installed and recently wanted it inspected. After trips to the Drag Strip and Road Atlanta the PG still looks like new. I think the PG is much better than some people give it credit for but if the WaveTrack had been an option when I got the PG it would have been a no brainer. The WaveTrack all the way.