I would imagine that most people never change the differential fluid or even think about it. Just like transfer cases on a 4x4, out of sight, out of mind.
Me, I'll keep my stamped metal covers. Don't need no stinking aluminum ones.
On Class 8 trucks, the only time the fluid gets changed is if there is a component failure and even then if there is no collaterial damage (metal in the fluid), it just gets topped off, not changed.
Finally Class 8 gearboxes are filled for life, no change needed.