I would also reckonmend lunging, as you can assess lexi's movements on the ground. Side reins, and a lunging whip should help. Just remember its not about speed, its about the extensions and elevation.
Im assuming you do english/dressage riding. I would be doing alot of 10m circles, asking her to cross her back legs and while her front is not moving much.
It may also be pain related? Bowen or chiro may be a solution.