I second talking to the vet. I see it as a quality over quantity of life. I don't know about rehoming her though, if she did not take the move very well then changing her family this late in life would not go well with her.

My DH had a Malamut and he was 15 (old for such a big breed) when I took him to be put to sleep. He was like your dog, not happy at all and just layed around all the time. He had lots of back anf leg pain too. The vet was only a short walk from our house and the day I walked him down there was hard, he really perked up and I walked in to get our old, in pain dog put to rest and instead I walked in with a dog that was all excited and happy. I still knew it was the right time. I don't know, maybe he knew what was up and was ready too.

Anyway, you know your dog best of all and if you feel that it is right then follow that. You are ultimately only ever doing what you feel is best for her.