I'm also partial to the word 'Guardians'. It's slightly more formal than 'Defenders' even though they both imply that they are protectors 'of freedom', and it doesn't sound like it's short for 'Department of Defense employee'.
I really dig the word 'Crusaders' as well, if nothing else because of the ire it would generate from all over the world. Not enough euphemism for common use, though.
Old school works for me too, especially if you dig into our own heritage. I'd be happy with 'Gunslingers', 'Cowboys' or 'the Posse'. Simple, and to the point.
If you really wanted to dig deep you could go with 'Legionaries', 'Cohorts' or 'Hoplites'. I think these have a good ring to them, but I doubt they would catch on.
"Guardians" does have a nice ring to it. I think either term (and lots of others mentioned) would drive some of the libs "over the edge" if they were widely accepted.