My interpretation of what happened is different than yours. Even though at the time, my interpretation, fears, etc, were the same as yours. Mine changed a few years ago when I started watching youtube (I think) video's, reading and discussing online, etc, and I saved some notes, etc, from back then.

My current interpretation: We obviously have active human imaginations. Our minds play tricks on us, our eyes see things in a fraction of a second that isn't really there-illusions. Real shadows caused by moonlight, streetlight, or some similar explanation (even pitch-black can have some light or our minds can "make-up" light). Without little doubt, these explanations probably can account for these experiences. The human eye and mind are easily fooled. Your comment, "Several times we thought we saw a large animal on the road, we'd stop and wait to see if move and the shadow of what we thought was an animal would go away" is evidence of that.

We developed to see/assume fear and danger in order to keep us safe/alive. This predisposition we have, I think in part comes from the fact that we come from a long and perfect line of people who had to assume things like a rustle in the bushes was an agent (like a lion, etc,) and not just the wind because the people who thought it was the wind and were wrong, ended up lunch meat and didn't pass on their genes.

So we had to become meaning makers and pattern seeking detectors. Great for survival, not so much for ascertaining truth and what's really happening in reality. Better to be wrong and alive! Over time we anthropomorphized everything (again, because we come through that long and perfect line of those types - ie/aka survivors that see agency in everything), even our feelings and instincts.

Add on top of that, we take what we've been taught, what we've seen in media, stories we've heard, our culture we're born into, our age, etc, and these experiences get interpreted by the meanings and explanations that have been offered to us during our developmental years. And these explanations, etc, keep getting handed down. If you had never heard of bad spirits at night, what cracking branches/sticks at night could mean, etc, those options would be less likely to be on the table for your mind to assume them to be. The same type experiences in the wilderness at night can come to mean different things to different cultures/times.

The further back in time you go, the less people knew about the brain, how it works, and the more people would rely on the folklore/memes/myth/archetypes understandings that had been passed on to them. An "evil spirts of the gaps" explanation if you will. Don't know how something works? Make an explanatory unfalsifiable story attributing it to unseen (except for when they are occasionally seen:) forces/causes. These stories/explanations/hypotheses developed and stuck long before we had the tools to properly examine them.

"A falsehood can travel halfway around the world before the truth can get its boots on."

"It's easier to fool someone than to convince them that they have been fooled." Same goes for mistakenly fooling yourself.

So I bet what you and I saw were actual shadows. Shadows aren't just flat images on a wall, they're 3D things that can move. If something gets between a source of light (moon, etc) and your field of vision, a shadow is created. And if that something moves, the shadow it casts moves along with it. Here's how the process works in steps.

Your eyes pick up something moving, because our eyes instinctively do that, and send the image to your brain for processing. Your brain's processing department sends that image to the memory department, where the image is compared with billions of images it has stored in the filing system. The memory department asks the processing center for more images, preferably something with details, and runs the images again looking for a match. It then sends confirmation back to processing, usually with a 100% match like "That's our mom, she feeds us things" or "That's a train, don't stand in front of it", but sometimes (more often than we think) it just comes back with a best guess due to lack of details. All this takes place in 1/20th of the time it takes to blink.

When the eyes don't pick up enough details, the memory does the best it can based on what it has in storage. If it was roughly the size and close enough to the shape of a person, that's what it says you must have seen. And since the image was no longer visible by the time someone says, "what the heck was that", there was no more data to process.

It happens all the time. Have you ever spotted someone you know really well in public, then looked again and realized it wasn't that person at all? Have you ever forgotten where you put your keys or your phone, and you swear you left them right on your bed but then you find them in the kitchen? And then, once you find them, you say "dang, now I remember". Again, that's your imperfect memory playing tricks on you.

Did these dark human-like shadows hurt you? Did they actually threaten you? No, they just showed up and eventually left without doing much more than just noticing you, if even that. That after the fact, shouldn't actually be scary.

Finally, it would make sense that you were so hesitant and fearful to not drive into the mountains as a teen late at night and that your mind was screaming at you to not go. "It was the middle of September, you had no flashlights at all... literally none of you were dressed right for a hike lets alone a 15 mile trek down a mountain in the middle of the night." You knew you absolutely had to get down the mountain because you had court the next day that you had to be to. The walk/hike down would be terrifying if you had to do it.

You went anyway cause your rational side probably secretly/subconsciously calculated the possibility vs probability odds and a cost benefit analysis of the worst-case scenario happening and correctly determined that while worrisome for good reasons, worse comes to worst, 9+ times out of 10, you'll probably make it out of whatever situation alive and you'll have a memorable experience instead of a night you would have otherwise mostly forgotten about forever. ...You turned out to be right!

This whole phenomenon reminds me of another related phenomenon I posted a long time ago- about being frozen still with a force/feel of pure evil when waking up in the middle of the night, which is actually: Sleep Paralysis!:) Here it is if interested:

https://www.cougarboard.com/board/message.html?id=17892067