Ha! Solved my own problem! The horse has non-zero Initial Values for several parameters- even a horse saved with the joints zeroed. Weird, right? Set the xrot and yTran values to zero, now the harness conforms properly. :) What I still don't know is why the problem hasn't shown up before, because I've worked the same way before and everything /seems/ to have been okay, although the hip did sometimes displace- an sometimes not. Well, I know how to fix things now. I'll leave the thread up in case someone else ever has the same problem. :)