2007 Ford Transit Van problems

Hiya I'm just seeing if you can please help me? If got a ford transit connect, I've have 18inch alloys on it for about 3 years now, I've just gone and had some second hand tyres fitted on the front, I went home and soon as I got to 40 mph the steering went mad? I went back and they fitted different tyres to it, soon as I got upto 40mph it did the same? They tried putting the back tyres on the front and it did the same? If got home and put my old 14inch alloys back on the front and it's doing the same? Do you think it's the tracking? Or the steering rack? Hope you can help,

Regards Steve