Hmm... the clicking makes me think cartiliage? Could be an old OCD lesion in the hock. Because she is a working dog, I'd play it safe and have a orthopedist look at her, or at least a vet that is familiar with working dogs. It would be awful if she had to retire early because she had an untreated lesion flapping around tearing stuff up.
Oh and being a mixed-breed does not clear her from inherited diseases... unfortunately. You are less likely to get weird breed specific, double recessive diseases like the bedlington copper thing or collie eye anomaly, because the chances are slim that both parents (being of different breeds) would carry the gene. But for common inherited diseases that many breeds carry, like hip/elbow dysplasia, cataracts, epilepsy, food allergies, etc, mixed breed are just as likely to get them. I have 3 purebreds and 1 mix in my house right now. The mix has HD, DJD in his elbows and corneal dystrophy... He used to be a SAR dog too, but had to retire because of his problems. *sigh*
I hope your girl has nothing big, and that its fixable.