I have a deep voice but it has a big vocal range, but when I sing in my "rock" voice (where it is all rough sounding, listen to Brian Johnson on ACDC's song danger when he's not singing high, that is a bit what mine is like) my vocal range is fairly limited, how do I improve it, as I say, when not singing in my rock voice I have a large vocal range.
There is a singing thread, I'll answer your question there. Go to musician talk.
Practicing as much as you can as loud as you can stretches your vocal cords, but I'm not sure about it if you're doing a voice that isn't natural.
Go with what rabid said.