A possible reason for hind end lameness is a torn cranial cruciate. This is common to the torn ACL that people get. It's very painful and the dog might resist putting any weight on the leg.
Another condition that might cause a dog to resist putting weight on a leg is patella luxation. In layman's terms, it means wiggly kneecaps.
There are surgical remedies for both conditions that can give the dog a great deal of comfort.
No matter what, you need to see the vet on Monday to find out what you are dealing with.
Let me know if you still have questions.