Someone (I believe know who, but now how) has done the following: 

Logged onto my wireless network from outside of my house, spammed my LJ with ridiculously childish insults, screencapped the insults, and posted the resulting screencaps to prove that I insulted myself.  (ETA:  this showed IP logging by LJ; another instance showed IP logging by a LJ Toy application.)

This has happened repeatedly over a span possibly as long as 13 months.  Our wireless network does not have a password (my parents think I'm being paranoid when I ask if we can secure it) and the broadcast range reaches two road fronts, at least four houses, and a wooded area behind our house.  However, there is no way for this person to go unnoticed for an extended period of time while she performs these hijinks, and on at least two recent occasions the nature of her stunts had to happen in real time, because she replied to posts in real time. 

Furthermore, this person lives about 20 miles from my home.  I have not seen her car in the area, nor have the neighbors.

So, is there some kind of device she could have hidden in the woods, which would broadcast our wireless to her at a third location?  And if so, how do I find it?

Thank you so much.  I know this sounds like petty childish crap, but this person is really damaging my relationships with friends and family, and this is the remnants of a much longer story, as I'm sure you can guess.  Thank you again.

ETA:  I had thought the IPs couldn't be a perfect match if different computers were involved, but have been told that is incorrect, and that all of the computers on the network would appear the same from the outside.

Using a productive network without any protections is careless. If a crime is committed using your network, even if you were not doing it but someone else, you might be guilty as well because of not securing your network. I am not sure if this applies to the USA as well, but I read about a few cases where this happened. Maybe you can use this to convince you parents to securing your network properly.

It is possible to forward your signals or using wide-range antennas, though as already stated by Ketchup, I too think that there are easier ways to get access. Additionally to Ketchup's questions you might do a HijackThis scan and a full scan of your system against malware.

Depending on how far the person is "threatening" you, you should consider police as well.

Don't say it's a calling me names on the internet. Say it's unauthorized access to my home network to do who knows what, including destruction of my reputation. It's all on how you sell it.

As for why to secure the network, start here: http://lifehacker.com/036577/todo-secure-your-wireless-network

Not having protection isn't always a defense. Cops will raid your house, not the people who steal the internet connection.

Might also check out the Oct 1st SANS' security tip of the day.
http://www.sans.org/tip_of_the_day.php