hang on there is a dog with similiar problems where the prosthetics have already been designed and made.
Your best bet would be to contact these people - I'll look the info up.
It was 3dsystems who designed these prosthetics - very good publicity for them and they already have the files.
http://rt.com/news/216235-dog-3d-printed-legs/

Probably wouldn't need that much change to adapt the same basic design to your dog.