Tim,
I gave a call to Steve Harris the service manager at Pacific Power Products, an Allison Dealer in Kent Washington. Phone 253-854-0505. They are well familiar with these type fails. They are in the process of proactively changing out 10 of these units on fire trucks in Seattle before they fail. He indicated that the heating and cooling cycles over time will cause a crack to form which allows the fluids to mix. In the reliability field that is called a thermal cycle fail which I have some experience with in semiconductors.
He said to look closely at the unit for any PN or build identifier if you cannot get help from Navistar as a part source. Steve at Pacific Power is having high quality units built by Seakamp Engineering for his retrofits. Basically you would need to take photographs of the existing unit with dimensions, end to end, and of the ports location and sizes. They can take that info and build what you need. Basically these are a vessel with many copper tubes that the transmission fluid travels through surrounded by the antifreeze. The construction is much like a heat exchanger in a boat. Seakamps phone number is 800-753-4436. Steve indicated that the ones he had built were $750 but it will vary a bit by size and configuration. If you cannot find the current PN, or Navistar is of no help, or if the source is out of business this may be a good path to follow. Steve at Pacific Power would be happy to field your call to discuss this if needed. Hope this helps.
Later Ed