Almost precisely, in fact--it's the same wrenched hind leg, pink and swollen and held stiffly out, that Gimpy suffered nearly a year ago. All I can think is that this must be a very common sort of injury for squirrels, which I suppose makes sense--the squirrel lunges up the tree, a predator grabs the leg in their teeth and yanks, and the squirrel gets a dislocated and puffy leg. The only other thing I can think of is some sort of snare, and I'd expect to see more footless squirrels in that case.
Having learned that squirrels can survive this perfectly well, I'm just going to leave extra seed out for Gimpy II. Once he heals, however, I will be completely unable to tell him apart from Gimpy I--it's the left hind on both of 'em--so the backyard wildlife will continue it's slow merge into a sea of crippled anonymous squirrels. But I suppose that was inevitable anyway.