My mare has a weakness in her hind as well we use lots of transitions to get her to push and engage her hind end in work, poles to get her to pick up and lift her hind legs. I find that the most effective thing has been lateral work it has really developed her back and quarters and has allowed her to hold herself better and move freer. Hope this helps :)
You can do cavelletti excersises
Quality and consistent work and nutrition will be the first step. Poles, gymnastics and hills are great ways to build on your daily work.
Hill work and backing. Trot poles small cross rails
Riding in an extended trot will build up muscle.
Lots of backing and rollbacks.
Trot poles and walking hill work
Backing and hill work. Worked great for my saddlebred gelding
Tons and tons of transitions. Lik walk 5 steps, trot 5 steps, stop, back up. You want to do these transitions only up to a trot to really build up his and once he has some more muscle, add in cantering
Lots of transitions, cavaletti or pole work.
Backing, teach her to drive/pull even just have her pull tires around, or hill work