Hi Lisa,
I can sympathise - my back seized up completely only 6 weeks ago. It's definitely stress-related in my case. Here are the things that have helped me:

In the immediate term and on a purely practical level:
- Going first of all to a Sports/Manipulative Therapist - after 10 minutes he diagnosed 3 facet joints that had been out of whack in my spine for at least 15 years and manipulated them back into place within minutes (with a few crunching sound-effects!). (I'm now 32 so I must have done something adventurous when I was in my teens and didn't realise the damage!) He then recommended me continuing treatment at home with some simple stretching exercises and the application of heat (ie hot water bottle or one of those wheat bags you put in the microwave). I know that we have a National Health Service and the US doesn't - but I was able to get this treatment privately for £20.

- Following up bone realignment treatment with a soft aromatherapy massage (available in most complementary health clinics) and then a few days later with a Deep Tissue massage - usually available through a Sports Therapist Clinic. This deep tissue treatment was invaluable and was able over 4 weekly sessions to release the ingrained muscle spasms in my back - they really hurt!
- Getting acupuncture and taking Chinese herbal supplements to boost circulation - also taking Omega 3 Fish Oil capsules and Glucosamine in the morning along with my vitamins.

Longer term:
- Have now joined a Tai Chi class - which really focuses on strengthening posture and lower back realignment
- Have just started the Level 1 Spring Forest Qi Gong course and am really feeling the benefits in terms of building up my muscle strength, lessening stress and increasing overall my sense of physical and emotional well-being. The meditations are also particularly effective in helping to build a positive, relaxed and open state of mind (the body usually responds best to this).

If you find that there are some significant emotional issues that arise when you are releasing stress, perhaps you might try EFT to work with the release of these emotions specifically - I find that it works quite effectively. (www.emofree.com)

Anyway, I hope that some of these tips might be of help to you.
Good luck!
best wishes
Ingrid :0)
(By the way, you might want to try chewing gum during the day to counter the rigid clenching of your jaw at night.)