Hey guys! Sorry to take forever getting back. My hard drive crashed!
Anyway, here are some more details -- he had a stifle injury when he was younger. He was basically on 3 legs for the better part of a year. I gave him a 1.5 yr break to get himself together and started lightly exercising him. He seems to be completely sound, but the only thing that gives me some worry is that he loses that side out from underneath him while I'm riding sometimes, so far only at the walk, but it just feels like when your knee gives out for a second, you know what I mean? Anyway, that and he seems a little lazy -- after receiving lots of comments on the laziness issue I think this old injury and the laziness could be related.
Another thing I didn't think of at the time I posted this video is that a few days before we spend the day with some friends in a deep sand arena, I'm thinking there could be some residual stiffness from working hard in the sand.
EDIT - We're still not sure what caused the original injury -- he just came out of the pasture one day on 3 legs. It was February in the midwest so very snowy/slippery, but upon x-raying the area, our vet assumed he may have gotten kicked.