Pray for sweet Rex

I am having a very difficult time trying to figure out what to do for Rex. I am very emotional while I write this because for the past 6 1/2 years, Rex has been the very best family dog I have ever known. He never barks or drives the neighbors crazy, he is sweet and patient with all the kids in the neighborhood, he is a best friend to Clint and Derek when it seems like the rest of the world is against them. At 103 lbs. he is an intimidating figure to strangers, and his presence makes me feel safe from potential seen and unseen dangers. He is always available to be a friend, go for a walk, or take a dive in the kiddie pool if anyone in our family ever needs some company.... Sunday he tore a ligament in his back leg that will require surgery. The cost is $2200. Needless to say that this would put a huge strain on our finances, and on my relationship with Jeff who has never really warmed up to Rex. Rex is not in pain, but he has no use of his back left leg. With his size and weight, it won't be long before his other back leg starts to have problems from overuse. I have been trying to get him to slow down, but he loves to play and hop around on those three legs. I don't know what to do. I have contacted Animal Sanctuary to see if they could take him, or help us out. I am ashamed as a dog owner to not be able to take care of him the way he deservers to be taken care of. I spent $170 at the Vet yesterday for his diagnosis and anti-inflamtory medication. We just are barely getting by as it is and we have spent the last 2 years finally paying off credit cards. I don't want to go back there again. Is that selfish? I know some people would say that he is only a dog...but he is also part of our family. So do I let him just hobble around until his other leg goes out...do I take out another credit card to pay for it and then in a year or so come up with another $2200 for when his other leg goes out?(Which 2 diffrent vets have told me that there is a 90% chance of this being the case). What do I do?