Some horses can have DSLD and be sound. Secondary arthritis/ringbone may become more of an issue than the ligament drop. A horse can have DSLD and be sound for years. My mare probably has had it for years before she started showing symptoms and even then the first vet who did ultrasounds missed it.
If it is DSLD- you want to pay close attention to her ankles and look for swelling/thickening of the ligaments. It is recommended to take ultrasounds and measure the width of the suspensory's for a true diagnosis. If there is swelling in both hinds I would be suspicious of DSLD even if one leg is worse than the other.
As the pasterns drop the legs will become more and more straight/ or camped under. So a horse that starts out with normal conformation will loose it as the disease progresses.
Here is a picture of advanced DSLD- notice how the hind legs don't have any bend in them at all. You can almost draw a straight line from the stifle, hock and ankle. http://www.naturalhorsetrim.com/Tree_DSLD_or_not1.jpg
And the swelling that eventually comes with it: http://i268.photobucket.com/albums/j...ltravis013.jpg
If you suspect she might have DSLD, I would modify her work load, as any tears (even micro-tears) in her suspensories will not heal normally- even if she is sound now. Dressage/jumping would probably not be a good idea.
It is possibly she will stay sound for years and it might not even cause any issues until her late teens/20's, or she could get worse quickly. Again no way of knowing. Every case I have seen has been different as far as age of onset, progression, and degree of lameness.