There is a reason that residency programs only offer one year contracts at a time (at least the ones I'm familar with). Just because the residency may act like they own you does not mean that's true. You have a right to switch programs. You do NOT need approval from anyone. The ACGME has already ratified this.
No one wants an unhappy resident. If you can somehow convince everyone that the problem is just a bad match between you and the program, rather than a bad program, few people will blame you for trying to switch. If your PD is particularly enlightened, he or she may even help you.
You don't need to have an 'external' reason, but it helps a lot. Everyone likes it (the old program and the new) if you say you need to move because of family/personal commitments, because it doesn't necessarily reflect poorly on your old residency program.
However, you can be honest and simply say that you're not happy at the program you're at. Unfortunately, may become labelled as a problem resident, a complainer or someone who can't commit.
Remember that programs with open spots need to have those spots filled, and the only way to fill a PG-2 slot is with a PG-2 resident. That resident has to be leaving somewhere, and there only so many people with good 'external' reasons for leaving. Transferring programs is not as much taboo as a lot of people try to make it out to be.
That being said, I recommend not telling your PD (or anyone else at the residency you don't implicitly trust) about switching until you've got a job elsewhere. This is tough because a lot of residencies won't accept your application for transfer until you have a letter of intent from your old PD. Make sure not to badmouth your program to anyone. It's unprofessional and makes you look bad as well.
Stay positive, and realize there are open slots that go unfilled every year, just waiting for someone like you to fill them.