I don't know much about technicalities but her singing has always been too aggressive, esp. when it comes to live performances.
Depressingly, I wouldn't be surprised if her technique has already killed off her voice in-between 2002 - 2007. She was quite open in saying how she was singing on Stripped was not the normal way she'd do it and quite extreme (the irony of it becoming the norm), it must have been a real trauma on her instrument (remember she went on an extensive tour in 2003 as well).
No wonder she sounded like a cheese grater by the time of Back 2 Basics.