I was told I need all 4 differential seals replaced because the fluid is leaking

Hello, I will go ahead and say you are extremely lucky to have the axle seals last over 216,000 miles! That is fantastic. I have had to replace one on my ML 350, and it has just over 100,000 miles. What I'm getting at, is these seals fail due to normal wear and tear, and no amount of fluid changes will prevent this forever. The seals are just a composite rubberized material that is squeezed into the axle shaft by a very very light spring. Lighter than a pen. When the axle rubs the material flat, or when the material wears a groove in the axle, the seal has to be changed. The reason no-one mentions it, is because this is an extremely common procedure. About as common as changing transmission fluid or a timing belt.