Welcome to the forums eubie5!
Time for a little helpful education then: The PTU is known as a Power Transfer Unit in an AWD system, and is important to send power to those rear wheels when its needed. For Ford and Lincoln, up to 50% of engine power and torque can be sent to the rear wheels (new torque vectoring vehicles can do up to 70%) and is done intelligently, as in the vehicle transfers based on wheel traction detection.
The problem with a lot of AWD vehicles between Ford and Lincolns is the PTU gear fluid gets neglected on being changed properly. Ford at first even coined this fluid as being "lifetime". There is no such thing as a "lifetime fluid". Lifetime means "till the warranty ends" among vehicle manufacturers.
The gear oil in the PTU starts to breaks down and loses its effective protection properties around 25,000 - 30,000 miles, or sooner depending on vehicle use and driving style. But many owners of these AWD vehicle will not have the fluid change and even mechanic shops will not change the fluid because "it's not suggested by the manual". So this 2012 MKZ at 60,000 miles probably did not get a proper PTU fluid change done...hence its fate.
Anyways, when the gear oil is worn out, the seals become more vulnerable to heat and friction wear. Eventually, the seal weakens and your leaking begins. The PTU is built "sealed" like the transmission is, and therefore instead of cracking open the case...its more favored to just replace the whole dang thing if the seal leaking gets too bad. Sometimes the outer seals only need replacement, but there are internal ones that pretty much doom the unit if they go.
The other thing is most units do not have a drain plug (I have heard some vehicles do have it) and require siphoning the fluid from the fluid fill plug instead, in order to perform a fluid change. It's not too hard to DIY if you know what to look for and have the right tools.
You will also have a RDU (Rear Differential Unit) that is a part of the system and helps manage power and torque going to the rear wheels. It too has gear fluid that should be changed around the same interval as the PTU for better lifespan.
If you get a brand new PTU unit, simply just keep this in mind to not let the vehicle's gear fluid go bad and you should be fine. The same goes for the water pump and coolant system.
Good luck on your MKZ should you choose to buy it!
Edited by Zalvern, 26 February 2017 - 04:58 PM.