Um, I didn't read all of this, so I don't know if someone has cleared this up yet, but stocking up has nothing to do with lameness or injury. Swelling is what occurs with an injury. Stocking up usually means the legs are fairly cool (sometimes a bit warm) and there's no lameness, and it goes down after about 20 minutes of riding.
Most of the horses at our barn stock up, and there's nothing to be worried about.
It comes from standing around too long. If you're really worried about it, see if you can get your horse turned out longer.
Now, if it's just one (sometimes two) legs that are 'stocking up', and it's hot, or he's lame on it, or it's really swollen, then that has to do with an injury, and you'd best get the vet.
But it's it all 4 legs (or just the back two), and the horse doesn't seem in pain, it's just stocking up from standing still, and will go down after walking some.