If this were me, after you had your first freelance gig lined up, give notice to Theatre X. Then, with permission, I would write a formal letter/email to production staff (PSM, Artistic Director, etc) stating what a great experience you've had there, etc etc, but that you feel it is time to expand yourself and work on a freelance basis (also that you would have no problems doing just that for Theatre X should they have need). This will start the positive word getting out that you're available and hopefully the word will spread.
Write another formal letter/email saying basically the same thing you told Theatre X, and send them to anyone in your contact list that you think will either hire you or would know someone who would hire you. Attach your resume so they don't have to ask for it. Tell them to feel free to pass the email along to whomever is interested. If you have Facebook, put a shorter version in your status update; do whatever on Twitter; post another update on LinkedIn (if you have one, if not - set it up!). Always have a positive vibe to the message, and you should get some interest.
Good luck!