I talked to my farrier and she thinks it's locking stifle, but she hasn't seen the video yet, so I'll update again when I talk to her, I just wanted to see what your opinion is too. :)
I'm leaning toward locking stifle too because the videos I've seen of stringhalt they really pick their legs way up and do it several times. He only does it once when he stands still. Once he's walking he's fine. It's only on his left hind leg too. He's due for a trim so it may be partly the club hoof, but it's mostly because he's a couch potato and because he has straight hind legs (post legged)!
Anyway here is the video... sorry it's of him grazing. My hubby is at work so I didn't have any help to walk him.
Here is the direct link if you can't view embedded videos: http://youtu.be/DIvukM6xfOM
I cut out the long grazing parts where he was standing still. So my farrier said if it is locking stifle I need to get him in shape! I've been planning to start teaching him to longe anyway now that I have a flat place to work him since he's almost four I don't have to worry about his growth plates in his legs being damaged from working on circles too much. Once he knows how to longe and knows the voice cues I'm going to switch to ground driving (double longeing) anyway so that will help too. The whole five acre pasture here is flat so I'll have plenty of room to move him around.
She said the easiest way to get him in shape without overdoing it is to follow the human marathon training method called from Couch to 5k. Check out this website for more information: http://www.fromcouchto5k.com/articles/training/the-couch-to-5k-training-plan/ I like having a plan because I'm so paranoid about overdoing it. Since I'll be starting out in hand (longeing, hand walking) I might just do it with him lol! I used to run intervals and was actually in training for half marathons at one time so this could be the excuse I need to stick with it.... or if I'm feeling lazy I can longe lol!! It's not fair to expect him to get in shape and not me though. :D