BILL—The first question is what does “wide behind” mean? By definition it is a lack of engagement. The horse spraddles his hind legs apart rather than stepping up underneath himself to carry more weight to the rear. Generally, this relates to a lack of strength and muscular development which would let him lengthen properly.
The first solution is to be less extravagant in the amount of stride you ask for. Better to ask less and get a correct lengthening than to try for more and run your horse onto his forehand.
The second solution is to do lots of strengthening exercises. We can talk hill work, but primarily the answer is in lateral movements that make the inner hind step up under the horse’s body, namely to do lots of shoulder-in. Combining short segments of shoulder-in, lengthen, re-collect, and shoulder-in allows you to “put your horse back in order” before he gets visibly disengaged.
A simple offshoot of this exercise would be to come out of the corner in trot and to leg yield towards the quarter-line, then make your horse totally straight and lengthen the stride for 10 or 12 meters, then re-gather and leg yield again on the same angle as before, once more interrupting it with collection and a brief straight ahead lengthening. This is the “staircase” which keeps reloading the hindquarters and builds the ability to carry himself in the right balance.