Thanks L1ftmech.
I edited/corrected my post below after sleeping on it. It was not the pinion seal that failed on the ZB Series, it was a retainer o-ring. Roadway did make a "home made" modification similar to what you described on XXB (-4/-8) Transmissions. The failure typically didn't happen until the truck reached >8k-10k hours. Roadway used trucks up to 40,000hours so they did this as a cost saver for them. Maybe someone out there applied this Roadway fix to ZB trucks?

FYI your a city or two off :-). Our plant was in La Palma, CA and the Product Support Facility was in Buena Park, CA.