The Ivecos I have encountered all had racks,yours having a steering box is different,it is hard to determine your next step,when the wheels are jacked up and they turn ok suggests all your balljoints,king pins etc are ok and there is no load on the wheels.The few vans I have encountered like this ended up being the pump or rack/box even though there are no leaks and no other fault visible. Good luck.
cdc, November 2014