Well, I think I've found out the reason for both Sunny's gaiting issues AND sore back. How lucky could I get?
Talked to the farrier about him having no shoes in the back and how that might relate to his sore [ness] and he said it is highly possible. Sunny would walk funny to get away from the pain on his newly-unshod hooves (which he isn't used to), stretching out muscles he normally dosen't use, pace because of muscle cramps, I bounce all over his back, and Voila! He's sore.
I recently had him shod because I will be camping in a park recommended to have all 4 shoes on. Yikes.
His back IS better though (I'd never go riding in the mountains on a sore-backed horse!). He gaited down the driveway in his handy-dandy little snaffle with a little coaxing, lol, and even tried to run down a hill so I guess he's feeling better :)
Changed saddles too. My other saddle was too small. We are currently riding Australian. LOVE it. NEVER go back!
Yay. Thought I'd share.