Thank you for the additional information. I really appreciate it.
Unfortunately older dogs do tend to experience a weakening of the hindquarters with some regularity...it's more obvious in some breeds (such as German Shepherds and Retrievers) than in others, but either way, it's hard to see your pet having trouble and not being able to do much about it.
It's hard to know if this is simple arthritis or if there is something more going on. Some dogs get spondylosis (which is a narrowing of the spine, which causes pressure on the cord that results in some rear end weakness), and some get osteosarcoma (bone cancer), both of which can present with rear end weakness.
Unfortunately, the only way to know for sure what you're dealing with is through an x-ray of her spine/pelvis area.
I understand that you(like so many other people) are in a hard financial spot right now. There are some organizations that might be able to help. If you'll follow these links: http://www.speakingforspot.com/helppayingforveterinarycare.html , http://www.theamericandogmag.com/non-profit/resources-veterinary-bills-financial-assistance, you'll find dozens and dozens of organizations that may be able to help pay for you to take your dog to the vet. Also, there are some clinics who offer free exams and other treatments and require that you only pay for medications. If you'll follow this link: http://www.vcahospitals.com/main/offer , you can see if there is one in your area.
As for making the decision to euthanize her. Although the only person who can really make the decision to euthanize your pet is you, I think the biggest thing you can factor in is quality of life. If your pup is in pain that you cannot control or alleviate, if your pup no longer does any of the things he likes to do, then that is the point in time where you must consider that not forcing the animal to linger on is a kinder choice.
I think that it comes down to is trying to put the dog's comfort above your own pain of loss. 9 is a very good age for a dog to reach so you have done very well by her getting her there.
At some point you may find the website http://www.petloss.com/ and the rainbow bridge story some comfort. Hardest thing in the world about pets is that no matter how long they live its really never long enough, but making this decision for your beloved pet is the ultimate act of kindness...to end their suffering in spite of the pain and hurt it will cause you to lose her.
If this were my girl, I'd check into the funding options I listed. Having a little more testing to determine exactly what is going on will go a long way towards helping you make a decision about where to go from here.
I hope this helps.