Kim Kardashian Gets Heartfelt Thank-You Note from Imprisoned Grandma

Kim Kardashian's mission to free Alice Marie Johnson is kicking into high gear ... and the 62-year-old grandmother has expressed her "deep and heartfelt gratitude."

According to a thank-you note from Alice to Kim, obtained by TMZ ... Alice was floored when she found out Kardashian had enlisted high-powered lawyer Shawn Holley to help with her case. Johnson is currently serving a life sentence for her involvement in a cocaine ring ... her first offense.

Alice wrote, "Ms. Kardashian you are literally helping to save my life and restore me to my family. I was drowning and you have thrown me a life jacket and given me hope."

Johnson also believes Kim's crusade will be a defining moment in history, and will show she was a "key figure in meaningful criminal justice reform becoming a reality."

Kim's not just focused on Johnson's case either -- she's also involved in the effort to free Cyntoia Brown, a Tennessee woman who killed a man who was allegedly using her as a sex slave when she was 16.

Kardashian and Holley are continuing to communicate with Johnson, Brown and their lawyers in order to develop the best plan for clemency.

And as Alice says ... "THE BATTLE IS ON!"