Hi Harv. Long time, no see. It's a small cyberworld after all huh?
Synchronicities have a simple explanation. The subconscious is a pattern matching machine at the service of the conscious mind. The conscious mind asks the subconscious to be notified when it finds a specific pattern, the subconscious obliges. It's simple as that, you can verify it for yourself.
What's amazing about the subconscious is that the level of pattern matching is apparently infinite. So it not only find patterns among sensory input, but also patterns among patterns, and patterns among patterns of patterns, and so on. The limit is basically set by the conscious mind, the subconscious will go as far as you ask it to go.
So when you develop an interest in synchronicities, you are basically telling your subconscious to find a specific pattern among patterns and tell you when they happen. More like asking it, "hey, let me know when you find two patterns of sensory data which seem related to each other when that should not obviously be the case". Complex and vague as that instruction might seem, your subconscious understands it perfectly and proceeds accordingly.
This explains the awareness portion of synchronicities, that is, the reason why you experience them. Of course it doesn't explain why they happen. Statistics takes care of that most of the time. Let me give you an example. Yesterday I went to the bookstore and one of the first books I saw was a biography of Joe DiMaggio. That was just hours after I read your post (you know which). Now, I read 5,000 different things a day, and the store has 5,000 different books. It's amazing that the subconscious can keep that amount of information and search for patterns among it almost instantly, but the fact is that it does. As my eyes scan through the shelves I have the feeling that I don't know what's written on the cover of each book but the fact is that I do, only I'm not aware of it. I'm only aware of the very, very small subset of patterns my subconscious feeds me, the ones I asked it to find.
John's synchronicities are also easy to explain. The Jeep, he obviously saw it on a subconscious level, just like I read the titles of about eighty books on the shelf yesterday but "magically" only became aware of the one who had any interest to me. As to John's coin, well, how many times in your life do you have the opportunity of finding an unusual object, or an unusual piece of information, that is somehow connected to a particular event in your life? I'd say hundreds of times a day. Most of the time it just doesn't happen (statistics), when it does happen you seldom notice (didn't ask your subconcious to notify about it).
People give mystic, philosophical, or cosmic significance to synchronicities simply because they don't understand how their subconscious works. But then that doesn't make it any different from anything mystic, philosophical, or cosmic.
After so long, it's still a pleasure to disagree with you :)