I think real is just a slang term for really. A long time ago, a bunch of teenagers decided that really would sound cooler if you left out the ly. And over time, real has become an expression on its own. So, I don't think they're completely interchangeable. In certain situations, you can replace real with really without significantly affecting the meaning of your sentences. In other situations, where real is part of a slang expression, it's a completely separate word with its own nuances in meaning.
Hey, dude, wanna go out tonight and get boozed?
For real. Let's do it. (for real here is an expression)
I'll hurt you real bad if you don't show me where you hid the money! (can be replaced with really, but the sentence would not sound as casual as it probably should)