Has there been long term success after replacing the PTU?
Yes. Replace the PTU and sell the car. No more trouble after that. That advice is for Edge, Explorer, and Flex owners. Ford has revised the box several times, but nothing seems to be helping.
There are theories on why the PTU will last between 20 and 80K miles, and my theory is heat. The PTU gets scalding hot, part of it from friction losses that any mechanic device has and the other part (speculation) is from it's installation environment. The cat is mere inches away from the PTU and we all know how hot these things get.
One thing we know for sure is that the synthetic lube installed turns to a black tar:
Now the first thing that pops in anyone's head is that the PTU should be serviced regularly to remove the oil before it turns to axle grease, and that makes sense. Unfortunately, Ford equipped the PTU with a drain plug for export models only. Yep. Now since this problem was known way back in 07 when they were crapping out in their first year of production, you would think that Ford would have immediately given all PTUs drain plugs and revised the maintenance procedure - but you'd be wrong. The fill plug is easy to get to even on the ground, but if you attempt to suck out the fluid with some sort of a vacuum apparatus, you will find that you can only get the hose about an inch into the PTU before running into gear obstruction (tightly packed box). Ford says the only way to service the PTU is to remove it, or I guess turn the car upside down....
This makes the Edge, Explorer, and Flex AWD systems light duty. Any towing in my estimation is going to aggravate the situation. If the vehicle is operated in flat areas in more tepid climates, there shouldn't be too much trouble.
My wife's daily commute includes a long drive up a hill/mountain, and of course the trip home requires another mountain climb. We also occasionally will climb Southington Mountain in CT, a full mile at a very steep 10% (all the way). If your car has an overheating problem, it will puke itself silly climbing this mountain.
My "solution" is twofold: I'm going to drill and tap in a drainplug at the bottom of the new PTU and change the fluid with every oil change and I'm going to instruct my wife to stay in the right lane on the mountain commute and keep her speed down to keep the cat from getting too hot. We'll also avoid Southington Mountain and take the old County Rd. which is much longer but a shallower ascent. I'll see how the fluid is doing and may extend the drain interval to every other change or twice yearly.