Answer:
The diminutive for Jacqueline is Jackie, which has the following variants: Jacqui, Jaquie, Jacquee, Jaq, Jacque, Jack, Jacq, Jacquie, Jac, Jaci, Jaki, Jacki, Jaqui, Jacky, Jackee, Jake, Jay, Jax, Jackie and Jakki.
Jackie is as noted a diminutive and not a true abbreviation.