Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your girl has had several orthopedic injuries which make supporting her rear end and walking impossible.
I understand that it is difficult to see her this way as in every other way she is happy and herself, and that unfortunately surgery is not an option for her.
Have you considered a canine cart? If she remains strong in the front end a cart to support her and allow her some mobility and freedom seems like a good way to proceed. It may be difficult to get her into the cart if she is a very big girl, but once she's strapped in she can move around and support herself and if you have access to the outdoors that doesn't have steps she can even get outside and relieve herself.
Here are some links to a couple companies that make very nice products:
None of these are cheap, especially for a large dog, but this is another option to consider.
Please let me know if you have any further questions.