I've fixed the threading issue I had. The solution came in the form of compartmentalization.
Before my change, I had a giant allCubes Dictionary that stored every single cube in the game. There was another Dictionary called _terrain that stored each chunk by it's Vector3 location. The chunk class had a list of cubes that cross-referenced the allCubes Dictionary. Sounds more complicated than it is.
Anyway, whenever I added/removed a cube or chunk I had to update that huge dictionary. When you have 2 million cubes in the dictionary, you're going to be referencing it constantly. This is why adding/deleting from it was causing so many issues.
I could have written a thread-safe dictionary or I could have used ConcurrentDictionary, but I felt that having the giant allCubes dictionary was a bad approach to begin with. By moving those cubes into the chunk class and referencing them from there, I've compartmentalized the areas that I'm constantly accessing. I can add/remove cubes from each individual chunk without bothering another one. That, my friends, is good business.
I didn't do this from the beginning mainly because I was short sighted. Simple as that.
I also fixed my texture issue. Switching my sampler from a linear wrap to a point clamp stopped the shader from reading too far out.
Now that chunk loading/unloading has been completed, I'm going to work on adding/removing cubes. I've been thinking about it a lot and I think I'm going to add a Dictionary called changes inside of each chunk class. I'll use the Vector3 position of the change for the key and create a change class that will store the type of change. That way I only have to save that changes dictionary and reference it when I rebuild the chunk each time. There's no need to store the entire chunk when I know it's going to be consistent.
I'm also going to keep working on the character controller. I don't know why but I hate programming a character controller. Everytime I go to start it I find something else I can do. Eventually I'm going to want to go past Version .01a though and I can't do that without a character controller.
So here's the latest terrain video. No audio but at least it's pretty.