There is alot of debate on " large breed" dog foods. There are several reasons for this debate. One is that the phosphorus and calcium is not as balenced in these foods more so than " regular food". Another debate was over feeding puppy food past 4 months, as puppy food is higher in protein causing the bones to grow even faster.
I will withold my opinions of Iams. I did note in your post, the lameness showed up after you started the food-and that is all I will say.. ( bites tongue and ties up fingers on the keyboard...)
Feeding the dog a better balenced, higher in quality nutrients, and then adding glycoscimine tablets at least 500 mg of that, I have found to work better. Using one of my current dogs as an example- while hottie doesnt have HD, he does have severe arthritus, and OCD. We can tell, even one day if I take him off glycosamine tablets, and obviously it helps. That and a good diet of balenced calcium including bone cartiledge has changed the original predictions of life span from 2-5 years old, to now he is 9 years old.
There are several better foods you can feed. Pick one, but " adult dog" food. Second add the tablets. Call your vet if you like on the tablets, but these tablets are very common in treating HD short of surgery. I am more suspect of the food balence than encouraging surgery at this point. Try eagle holistic. Is along the same price range- and much better in quality ingredients.
Here is a link for the tablets I am talking about.
http://www.drsfostersmith.com/Produc...&N=2001+113677