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Well I can't honestly tell you what's wrong for sure. But I read in a magazine that if your horse needs to pick up its feet you can try using grids and combinations, and try bounces, or in-and-outs. Your horse learns that after he jumps the first one, he has to go right in and jump the next one. If he doesn't pick up his feet it won't be pleasant, to say the least ^^;

That's all I've got. I can try and do some more research ;]

What do you mean by prop? Do you mean like chip (get in to the jump too close, and pop on the front end last second before jumping)? That could be part of the problem in the hanging legs. How long have you been jumping him for? How old is he? Is he uphill to the fence? Or does he drag you there? One thing you could do is set up canter poles to keep him striding right all the way to the jump. Poles help keep them up. Is he doing it with both leg, just especially the left? Or is it just when he's on the one lead?

Excuse me? How do YOU know I'm not a jumper? I did TONS of jumping back in the day, and even back when I started with Julie. (Had to stop, Bobbin went lame at a show because of slippery landing, and no one else was to be ridden because they were ponies. XP )
If you don't want to Die, Then don't jump. Don't get on a horse. It's that simple. Horses are dangerous animals. You know that. When you get on and start jumping, you know your horse could seriously injure you from anything.
A canoe is no more then half a foot high. You could try poles over it, try raised trot poles and such to get him to pick up.

Solid jumps don't make a difference in this situation.
Do you have a video or pictures you could post? oh! and does he doit over oxars too, or just verticals? If you ride to the fence with solid leg, and keep him from propping, does he still do it all the same? (We need to figure out where the issue is first, if its him chipping, he could be hesitant to really use his hocks). I don't know, since he's been checked by the vet, that's the only other thing I can think of that would make a sudden change like that. I'll ask my coach about it when I see her.

One thing I try is set up a one stride then bunny hop, so like your first jump is your one stride leading into your bunny hop. But I wouldn't recommend anything to high at first. And maby try him with canter poles :3 *I know it's already been said but* get him thinking.

I agree with Brat though, if you had a video, that could help alot, and I also have to agree with Antlers. xD If the different things don't work out and he's just not getting it, maby try giving him a brake from it for a bit, so when you go back into it, it's a little more interesting. :3 Personally I like LOVE jumping so every other season might be a bit hard, nya xD But even if you leave it for say a month *I know at least down here show seasons off.* it could help a bit :3 So far it's worked with some of our guys. Sometimes all they need is a little change every once and a while, but namley just try and make things interesting for him, but also more of a challenge (Like was said quicker turns etc.)

If you have someone who is experienced and knows how to do it properly, rapping can very effective to get a horse to tighten their knees back up. However, if you don't have someone who really knows what they're doing, you can also easily hurt your horse, so I wouldn't try it unless you have someone who can help. Done properly, it's not cruel or abusive, but again, you have to know what you're doing. I've also heard about little stiff rubber spike-type-things that you can put on the top of the jump that serve the same purpose. Never actually seen them myself, but it was recommended when we had a horse that started knocking rails.
What really worked for him though (and it may not be helpful for you since you didn't say anything about him touching the rails, just dangling) is we replaced the nice friendly round poles with 4X4s. They're heftier, more solid, and if they hit them a lot of times they'll rotate in the jump cup rather than coming down, and if they hit them a second time they'll get them right on the edge. We all started jumping with the 4X4s because we were in a group jumping lesson, and Tovaar wasn't allowed to jump with anything else since he seemed to be amused by knocking over as many poles as possible. They'll still come down if you hit them hard enough, but it packs a little more punch if they do hit them.

Other than that, I agree with what Antlers said about doing shorter courses with harder questions. Not necessarily higher fences, just trickier course that will keep him thinking and keep him interested. You might try shortening the distances in combinations to make them a little tighter, which would make him get deeper to the fence and can help to neaten his frame, but that's just a guess in the dark (which I know you said you didn't want... sorry)

As a note, I know I am primarily a dressage rider anymore, but I did used to event, and when I come home my trainer often has me get on one of the kids ponies and jump them around to work through an issue. I'll admit I don't know as much about the more technical aspects of hunters, but I do know a bit about jumping, and although I never rode about about training level, I was always on the ground helping my friends train, who were doing Prelim/Intermediate eventing and 4'3 jumpers (the horse I mentioned earlier). I don't claim to be an expert, but I do know a few tricks.

Going and jumping him outside of the arena is a good suggestion, if you can find somewhere to do it. And I know you said you had a vet check him out, but sometimes vets can overlook things, so maybe get a second opinion?