My ex has custody of our child but we do have a parenting plan. She wants to move out of state. How easy will that be for her?

There is a process for both parents to go before the court and have their arguments heard. You can begin by filling out the forms I have pasted a link below;

http://www.courts.wa.gov/forms/

down at the bottom left hand side there are forms for child relocation. If you have difficulty with the forms you may seek the assitance of a courthouse facilitator or hire an attorney in your area.
Good Luck and remember this is decided in the best interest of the children.