Your bed is not aligned correctly.
You should put some information about your ST in your profile, like the year it is etc. It makes is easier for people to assist you.
I had an 03 with the mis-aligned bed, it was so bad that when I turned left the right tire would rub on the inside bed fender. I had it fixed under warranty. Others on here had the same problem and they loosened the bed bolts, shifted the bed over and tightened them back up. You can measure from your cab out to the edge of the bed on either side to see if this is indeed the problem
