Sole custody is you have the say to pick the school the child attends, dr's ...stuff like that. A move away is a little different. Does it say anything in your orders about a move? Ours states "no more than 30 miles without court approval". You will have to go back to court to work out a new visitation plan and will be responsible for the transportation costs since you are moving. Could the dad fight it? yes...you will have to prove why it is in the best interest to have the CHILD move away from the father IF he is exercising his parenting rights now. I would send him a certified letter
stating your intentions and what you would offer to him as far as visitation/transportation costs. (meet half way? airfare? long weekends from school? more time in summer? winter and spring break?) See how he responds. If he agrees, then you can just have it written up :) Good luck!
PS Not to many people come to this board. You would do better to post in visitation issues on the main board :)