The advice about not changing trans fluid only applies to cluch/band based automotive automatic transmissions. What happens if you do not change it for a couple hundred thousand miles is the friction material ends up in the fluid, and this still allows it to somewhat work until you put new fluid in. However, at this point, the transmission is already on borrowed time, the old fluid is just masking the issue. And, if you change the fluid regularly, this should not become an issue in the first place, and is the best option. I do 50k intervals on my personal pickup.
But, like I said, this only applies to the light duty automatics found in cars and pickups, and beat up ones at that. The only heavy duty trans this may somewhat apply to is the Allison autos. But otherwise, differentials, transfer cases, and manual transmissions (and automated manuals) in any vehicle, from a car to a semi, will only benefit from an oil change (provided the right fluid is used).
In your case, milky fluid means water ingress. Change it ASAP, this milky fluid sucks at lubrication and will cause rust. I would also change the gasket on the diff (or use RTV) and check the vent/ breather tube on the diff, maybe add a longer tube if you go into water often. Also, I would regrease U-joints and other chassis components (if they are greasable) and check the trans and motor oil as well for water.