Our boy was "down on his hocks" when we first got him, and didn't stand straight at all. The chiro helped a lot with that. A lot of feet and knee issues can come from the back. I don't exactly know what you mean by locked hocks though?
As for the limp, it can be caused by any type of misalignement in the neck or whithers. In my dogs, if it's combined with licking of the forepaws, then I know their C7 is out pretty bad. Other vertebrae will give different symptoms.
For a limp, there are standard rule outs. Pano for growing pups, which I don't know much about, but I think that it's not just one limb? Tick diseases can do it also.
I wouldn't use a chiro that hadn't gone through the avca training, which is pretty extensive. here's a link to their site:
Not all chiros or vets will be on the list -- some don't pay the annual fee, etc. Depending on where you live, it can be tough finding one close to you.