I have an Aussie that we rescued from near death. She had been horribly abused and did not know how to play either. We do have another dog, and it was a long process, but my border collie was eventually able to draw her out. Have you tried any tug toys, like the knotted ropes? At first our Aussie was scared of them, but now she loves them, not to mention tennis balls. Some things they never get over (like loud noises), but just keep giving her lots of love.

Good luck (and bless you for adopting her!)