Thanks for the comments guys! Chrome is fine. This is actually a really mild episode of his stifles locking. I'm just being extra cautious since I've been riding him quite a bit for the challenge. His health comes first.
The ground work and physical therapy have been really helpful. He didn't lock up (no toe dragging) at all today and wasn't sticky with every step while grazing like he was a couple of days ago. The only time I really noticed it was during tight turns (they didn't give out, but he slows way down and acts like he's expecting it to happen) and when asking him to pick up his hooves (he has to shift around to unlock them before I can pick them up... It wasn't happening when moving forward or sideways, only when picking up).
I did his stretches and tail pulls, then let him graze while I brushed him and applied his neem oil. Then I decided to try backing. Most of you probably know by now that he has trouble backing. He seems to lock up worse going back than forward which is weird because most horses use backing up to unlock. Today however, he did great!
I held his halter while standing directly in front of him, asked him to lower his head and back up. We were on the soft grass with a very slight downward incline. Very slight. Barely noticeable lol. When I asked him to back up he hesitated for a split second as if he thought he wouldn't be able to and then started backing. When he realized it wasn't going to lock up, he backed confidently, quickly and energetically with perfect diagonals. I've never seen him back that easily and quickly before!! He normally drags his feet backward, but today all four were easily clearing the ground. That made me so happy! I think the PT is making a big difference. I'm going to have to keep it up even when I start riding again.
I'm doing the walking and stretches every day, but doing the tail pulls every other day. For the walking I'm holding the rope in my far hand, facing him and flicking the end of the rope at his haunches so that he engages himself. When I lead him normally he steps his hind hooves in his front prints with his hind toe only about an inch in front of his front toe print. When I ask him to walk out he steps his rear hoof in front of his front print with about three or four inches between the front toe and rear heel. That's quite an overstep!! It's important though because the lazy walk does nothing for him.
As of right now I'm hand walking him on the driveways. I figure a break from the pavement is a good idea right now.
One other thing. I forgot to mention; when I rode Rocky the other day Chrome was hiding from flies in the barn out of sight. At one point Rocky neighed for Chrome. Chrome didn't answer. I was worried Rocky would get upset if he didn't know where Chrome was, so I called Chrome's name. He immediately neighed and walked to the gate to see me!!!!! How cool is it that my horse will answer to me and not even his pasture mate????? Chrome also ignores my husband's and dad's vehicles when they get home from work (we live on the same property but different houses), but when I drive up he comes to the gate and neighs at me! I love him!!