Joint custody implies that you will agree on major parenting decisions. Where the children live would definitely be considered a major parenting decision.
The usual boilerplate language for relocation is that you can't move the children out of state without consent. 100 miles might make sense to you but state lines make more sense to the legal system.
I would strongly recommend that you have your agreement stipulate that parenting decisions on which you cannot agree be decided by an independent arbitrator. Arbitration is quicker and less expensive than visits to court. Many of these arbitrators are child psychologists and thus do a better job of advocating the child's best interests than a judge and a couple lawyers can.