The horse is not standing square, so making a definitive call can be difficult. But it looks like you've got a couple of conformational issues, here.
First, looks like the horse is "sickle hocked." This is a common fault in Walkers. It is believed by many breeders that a curved rear leg will allow the horse to "reach under itself" better and that will give more overstride. In fact that is true. But it also means a weaker rear leg and may be major contributor to the large amount of rear end issues that plague Walkers.
Second, the rear leg is quite long. This, again, is not uncommon in laterally gaited horses and is not all that much a concern as long as it does not unduly affect movement.
If you've got "forging" that means that the front end is not moving fast enough to get out of the way of that long, curved rear leg. Some advocate the rear leg moving outside the front foot. This would not be a good idea, biomechanically speaking.
Talk to your farrier. It's likely that you'll have two broad options.
First, speed up the front foot. From the photo it does not look like you've got the "long toe, low heel" going on (as many Walkers do). It does look like the angle might be a bit low and if that is the case then get the farrier to "stand the horse up" by keeping the toe short and growing more heel. This is allows the foot to break over a bit sooner and that might be all that is required. A second option is to roll the toe to get a slightly earlier break over. Squaring the toe might also be an option.
Second, slow down the rear. Here the toe can grown or shaped to extend the time to break over and this will allow the front to get out of the way.
It's even possible to do a little bit of each.
Riding style might also be an issue. In nature the horse has about 60% of its weight on the forehand; when we get astride we want to move weight back to the large muscles of the rear, and have about 40% on the forehand. If you're keeping the horse on the forehand (i.e., not riding such that the weight is shifted) then that extra weight will alter break over. If you're riding "feet on the dashboard and butt on the cantle" then the horse is ventroflexed and that prevents the shift of weight to the back. If you're using a long shanked bit and "setting the head" with your hand you're also preventing any lightening of the forehand. Have someone video you as you ride and then review it with an instructor/coach who can intelligently evaluate your seat and your use of the aids.
If you've not read them go to Amazon and order Dr. Deb Bennett's three books on Principles of Conformation Analysis
. They are about $14/ea. And well worth the cost. While they mostly focus on trotting horse conformation they still are about 98% accurate when dealing with gaited horses (she currently rides an SSH if memory serves).
I don't consider "forging" to be a minor problem, because it is often the "canary in the coal mine" and a signal of larger issues. But because it can be the result of so many issues it can be difficult to address.
Good luck in your project.