You don't necessarily need more than one mic to get a good distorted guitar tone.
What you do need, however, is to make sure you are evaluating your tone against the mix of the other instruments. Sometimes a guitar will sound like dogshit on its own but great in the mix with drums and bass.
Here are some things you should try (some of this has already been mentioned).
-Get the amp off the ground. Put it in a chair, tilted upwards if possible. This allows the tone to be dispersed more evenly.
-Record in a closet, preferably full of clothes that help absorb some of the sound.
-Place the sm57 so that it sits at a 45 degree angle from the edge of the amp about an inch from the mesh. Then turn the mic inward so that it points about an inch from the edge of the cone.
-Back off the gain, particularly if you are playing thrash-type speed metal. Power doesn't come from the guitar or amp, but from your wrist.
-Play full chords where appropriate. Don't play two-note octave chords because it sucks out tone.
-Make sure the strings are new.
-Manually double your tracks. Good distortion comes from playing the part more than once. Simply copying the track and delaying slightly won't cut it.
-Lean towards getting more of a midrange sound to your tone than what you initially want. Overaccentuating the mids will cut better through mixes, and you can always cut back if necessary later.