If a hot differential is cooled rapidly, by going through water for example, a low pressure area will be created inside the differential. Oil seals are designed to seal in one direction only, so if the low pressure inside the differential cannot be equalized quickly, water will be drawn past the oil seals and into the differential, which can have serious ramifications.
You would fully expect that a company as large as FUSO/Mitsubishi would invest a considerable amount of time, effort and money into the research and development of their components in order to produce high quality, durable vehicles. I would too, but I must admit that since buying my truck I have questioned this on several occasions.
Unlike all of my other tanks, which are made from stainless steel, the grey and black water tanks are made from plastic; Polyethylene to be precise. Surprisingly, there is not a significant difference in price between custom made stainless steel tanks and custom made plastic tanks, but the 10mm thick plastic tanks are significantly lighter than similarly sized stainless steel tanks.