hmm. I agree with most of the above.
Your horse is heavy on the forehand. Naturally, a horse caries 60% of weight on his front, 40% with his hind end. Engaging the haunches means the horse is stepping under himself with his hind legs and taking on more weight, creating a balanced horse (in simple terms). I recommend transitions, hill work, cavalettis, and circles. Jumping will also improve muscle mass in the haunches. More then likely, your horse is not developed enough to carry the weight you desire on his haunches. Tight circles are also important to increase balance. Asking your horse for longer strides should be done, but lengthening and extension won't be achieved until balance and engagement have. When you have a relaxed, forward and active hind end, you will see the tail swing, feel the back move, and when you look at the hoof prints of a trot, the back hoof should touch the front hoof.