Place the boneless skinless chicken breasts in the bottom of the slow cooker.
Add the diced onion, minced garlic, dried Italian seasoning, salt, and black pepper over the chicken.
Pour the marinara sauce and chicken broth evenly over the ingredients in the slow cooker.
Cover and cook on low for 3 hours.
After 3 hours, remove the chicken breasts and shred them using two forks.
Return the shredded chicken to the slow cooker and stir to combine.
Add the uncooked ziti pasta to the slow cooker and stir well to submerge the pasta in the sauce.
Cover and cook on low for an additional 1 hour and 30 minutes, or until the pasta is tender.
Once the pasta is cooked, sprinkle the shredded mozzarella cheese and grated Parmesan cheese evenly over the top.
Cover and cook on low for 10 to 15 minutes, or until the cheese is melted and bubbly.
Sprinkle the chopped fresh basil over the top before serving.