Just wanted to let you guys know that I had the same problem as the OP, my steering wouldn't fully return to center making highway driving very annoying. \
We adjusted my steering box, loosened the hex bolt by a little over half a turn, now it works much better! It was a bit annoying to get to the adjustment, we had to take out the air filter, battery and battery pan, there really wasn't enough room from the bottom for us (maybe someone is more ninja than I am?).
Hope this helps in some little way for someone.