I would recommend GHR, sled sprinting, and speed sets for sprinting. Or you could find a slightly inclined road and sprint up that. In my opinion split jerks work really well in getting more explosive hips and faster footwork, just make sure you can do them right.
First off I would ask Cressey about this, but to me it sounds like you have poor hip flexibility and you lower back is the weak link in your posterior chain.
In my opinion, sumo's don't hit the hammies as well as conventional. I always feel them in my adductors. If you want to jump higher, get explosiveness in your quads. Tuck-jumps (weighted or BW) and just jumping on high plyo boxes worked well for me.
Ask some of the professionals here, it's extremely good advice for free. They could answer it better than we can. There was an article about 2 days ago that adressed a lot of speed/athletic work and drills, called 50 More Tips. I thought it was a very good read.