Place the chicken breasts at the bottom of the slow cooker.
In a medium bowl, whisk together the pineapple juice (drained from the 20 oz can, reserve the chunks), brown sugar, soy sauce, garlic, and ginger.
Pour the sauce mixture and the reserved pineapple chunks over the chicken.
Cover and cook on LOW for 6 hours or HIGH for 3 to 4 hours, until the chicken is tender and reaches an internal temperature of 165°F.
Remove the chicken and shred it using two forks. Return the shredded chicken to the slow cooker.
In a small bowl, mix the cornstarch and water to create a slurry. Stir the slurry into the slow cooker and cook on HIGH for 15 minutes to thicken the sauce.