The diff oil is changed every 500 because it's the same oil for the brakes, Mobil 424 has friction properties which brake down over so much use, not replacing is can result in premature brake wear and loud brake grinding sounds, I assume your using the recommended oil?

As for your diff issues, I can't say I've had any problems with the LC trucks, I've certinly seen Hyundai diff problems though.
But the same issue with 3 trucks, I would look long and hard at what your customer is doing with these trucks and get him to change it if he can.
Also having "luck" with Hyundai's tech support is like winning the lotto