Yup, probably a control valve problem or an internal box failure. You can pull the control valve and dismantle it fairly easily. Make sure the spool valve moves nice and freely inside it's bore. As bb400dart mentioned, make sure you mark the position exactly where the valve is mounted on the box so it can be return to that same spot or you'll have to readjust it.....which can be a messy, time consuming job if you haven't done one before. The difference of 1/32" can mean the difference between perfect and a hard pull to one side.
