It is possible for your pet to have a condition called IVDD (Intervertebral Disk Disease). This occurs when the disc presses on the spinal cord causing pain. Check out this link for more info and see if your pet can relate to any of the symptoms:
http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=3223
Many years ago my family had a little mini poodle (about 17 pounds) and suddenly she became _paralyzed_ in her back legs. We were terrified.
We live not far from an amazing vet hospital, (Univ of Pa) and she did need surgery as I believe it was two discs (now I wonder if it was vertebrae?) that started putting so much pressure on her spinal nerve it would have fully paralyzed her if they hadn't taken them out. She did very well and lived a full life after the surgery.
I have several pets and struggle to pay bills as this economy is killing me, but I encourage you to see if you can find a good quality place that will work on a payment plan with you. The condition our dog had was _RARE_. I don't want to scare you with this, but it is possible.
If your dog is _not_ paralyzed but stiff or struggling, I would think it is not the condition our dog had. I have a beautiful german shepherd who will be 14 years old on the 18th (this Tuesday), and now his hind quarters are weak. Accupuncture does amazing things for him, as does a drug called ADEQUIN!!!! It is incredible if your dog's real problem is a form of arthritis.
My 'bottom line' advice is to not give up on your beloved companion. When my beautiful shepherd first had trouble with his hind legs, he was 10, we went to the vet, and the vet told me he had only 1-6 months to live because he had 'DM' and it is 'common' among big dogs especially shepherds. I was devastated. Well, thank God, my vet was completely wrong. He was basing his diagnosis on assumptions that had nothing to do with my particular dog.
If it is a joint-related issue, there is a company called 'liquid health' that makes 'k-p glucosamine' called 'level 5000' for _big dogs_ and it changed my dogs life!!! You would need the big dog formula, and believe me, it is incredible and we were able to take my best bud off of NSAID medication for almost 3 full years just by using that supplement.
So, be hopeful, and search for solutions. I hope they will be there for you.
Best wishes.