Hi Lisa,
Welcome to POMS. My situation is slightly different than yours, i have sole custody of my 2 children but they go to their dads every weekend. I contacted my lawyer for advice when my new partner and i decided to move to Oz and she said that i needed him to agree and to make it legal. I spoke to my ex husband and he finally agreed to let them go. My lawyer made up an agreement which we both had to sign and there shouldn't be any problem now (apart from the fact that my ex has now announced he is planning to follow us out there once we are settled)
Not sure how it will work for you because you both have joint custody, have you told your ex yet that you are going?
Maz