I don't know, I would say viaducts and overpasses are both just a type of bridge. The PA 43 and I-476 examples in particular are constructed like any other overpass (with beams, not arches), just much taller and longer than usual. I wouldn't have a big issue with calling either one a viaduct, but I still think they should qualify for the thread.
That just depends on what one considers to be interesting about the height. To my mind, a structure built merely to cross another road that happens to be very high is more striking than one that is built to cross a valley, because I'd expect height of the latter but not of the former. This does seem to fit the spirit of the thread: as I read the OP, it talks about highway grade separations that happen to be tall due to surrounding terrain. To me this connotes something distinct from a structure that is tall due to surrounding terrain, and that happens to cross another highway above grade.
But in any event, that's a separate question from what distinguishes viaducts from overpasses and the like. It's true that they're both types of bridge, so there's no controversy between our statements there. But I'm not aware of those terms being defined by any method of construction. A certain method may be typical of one or the other, but the method isn't a defining characteristic. (See also the terminology of rotaries vs. roundabouts and their kin.)
To put that more clearly, you might build something out of beams
because it's an overpass. But it wouldn't be an overpass
because it's built out of beams.