The ptu updates have nothing to do with any facelifts/midcycle ehancements/minor model changes. I would assume the changes below apply to all explorer ptu's, but I haven't physically checked them to be sure the retail units were updated on the same timeline. Fill/drain plugs are a different story. I haven't seen a definitive chart/timeline identifying which ones have them and when they got them.
The power takeoff unit (PTU) has been a service concern on some PI Utilities. There may be an excessive oil smell when driving, the AWD may not work properly, or the PTU housing may be cracked, leaking or noisy. The cause of the PTU damage is the excessive torque loading of the idler gear, which causes the bearing to walk (move) into the housing.
In January 2014, a Full Face Thrust Washer, FFTW, was introduced. This new washer was designed to prevent the idler bearing from walking into the aluminum case. In October 2015, a second change was made, in this case a loose-fit idler bearing. In dynamometer tests, this bearing design change showed a twice life improvement over the FFTW design.
In June 2016, a third change was made. In this case, a new bearing design, one without drawn cups, was installed. This eliminated the walk mode of damage. Dyno testing shows a three to four times improvement over the FFTW design.
It seems that underfilling was common and my have continued into 2017 or even until today - only way to know for sure is to drain your fluid and see what comes out. They can hold about 18 ounces - underfilled units only had around 10-12 ounces in them.
Stop wondering. If it was replaced prior to 50K, it probably was done under warranty and your dealer should be able to look that up and confirm. If it was customer pay for some reason, you would probably have to look at the part and see if it has any markings that might indicate it was replaced at some point.