If it is under your feet it has to be the sway bar bushings, that's what they do when they get real bad. Feels like someone is hitting the floorboard with a hammer!! You have two choices, replace the bushings themselves with ones from ES, cost is about $35, or you can just order two new end links from Ferd. I bought mine at
Ford Parts and Ford Diesel Parts at Powerstroke Shop cost was around $45 each. I am warning you replacing them is a real pain, not getting the new bushings in, but getting the old ones out!!!
NCH