For stiffness I would read up more on real anatomy. Become at least familiar with the skeletal structure and how it moves. I'm sure you can find some tutorials on that around DA.
Actually, one of my good friends in the comic industry told me a good way to improve is to get a sketch book, hang out at a park and draw everything you see (including the people.) It's frustrating at first but the practice is very good. At least it was for me!
So you think more practice with the human figure would also lend dynamicism to animals? That's interesting. Most of what I practice is human anyway, but more in-depth practice (especially life-drawing) should definitely help. Thank you!