I have one that is also stiff behind. Her conformation isn't bad, but she's a little high butted and straight legged (not quite post legged, but straighter than I'd like). We have spent two years trying to figure out why she's not moving well (or like she did as a 3 year old, when she moved much better). She's had surgery on both stifles for stifle lock and recovered well.
Here's what we've found - she's incredibly tender footed. Any hard ground, gravel, etc. makes her "ouchy". I've had big horses like that and we just shoe them, but can't shoe the mini. So the more she's worked, the more she shortens her stride because her feet hurt, and tightens up her hip and back. She actually at one point had a rounded back from inflammation that we couldn't figure out where it was coming from. After lots of "try this, try that" here's what we found works.
She gets Masterson method massage weekly during show season, and the day of the show, special emphasis on her hip. Worked either in shoes (build a bear) or in sand only. 1/4 gram bute to show, if the show arena is packed, or has any rock in it (sand is ok).
She drives, jumps, halter obstacle, showmanship, etc. She's great for small kids because her attitude is incredible. She's not National quality for driving, but she can hold her own at the local AMHR shows, and in fact is only 7 points from her WCP Hall of Fame.
Just some thoughts. Every horse has conformation flaws. If they can work comfortably, to me that's what's important. I wouldn't want to have them doing something that puts them in pain, but if I can alleviate the pain and not cause any damage, then that horse will have a job, and more importantly, a future.
edited to add - She did make 4th place in youth WCP 13-17 under on one card at Nationals this year, so it isn't as though she's not a quality driving horse, just not quite where she's going to come in the ribbons.
