Yeah, just Google Ford PTU stuff and you'll get yourself sick. Ford won't typically don't it either as they will just remove and replace the PTU.
The problem really lies in, IMHO, the low amount of fluid inside of it. It takes alot of abuse and should really have more. You may be just fine, even at your high mileage, but it's hit or miss with these things. My gut instinct is telling me that it's very dependent on how you drive. I've got no science or data to back this theory, but my hunch is that when that oil exceeds a certain temperature, it begins to lose some of it's properties and that's the downward spiral...the weakest link inside that case is the idler gear bearing, hence why Ford added a cooler to the side where that bearing sits. It's helped some but once that oil goes bad, things go into a domino effect. And if you drive it hard, this is what likey accelerates the process. That PTU is just basically a gearbox directly connected to the transmission. It is always spinning when driving, regardless of how much power is being transferred to the rear wheels. It's the RDU that makes the adjustments as it is the component that has the clutch packs.
That's why I've always felt they need to add more capacity to it or get a legitimate oil cooler/heat exchanger on it.
So, short answer. Frequent oil changes will mitigate many of the failures, regardless of what a mechanic shop says...