it might just be the way the pictures are angled...but to me it doesn't look like she has much muscles in her hind-quarters...but if you say there is, I'm sure there is
Collecting properly, head on the vertical and hind-quarters enaged. Alot of long and low work...if you are comfortable just ride her in a halter and lead rope for reins. Use the lead rope only to do emergency stops and see if she'll keep her head down low. Alot of flexion...circles, turns, anything where she is using her neck.
For the hind end, try some trotting up hills, or collection work.
Yeah her head is like beautiful *drool* lol