The VSS, for some reason XL's seem to go through speed sensors more offten the twin cams, I've replaced quite a few in XL's, some time small filings stick to the sensor and you can clean it up and it will start working, but I would just install a new one if you find that's the problem. Look thru the self help section on speed sensors.
If your speedo is not working the turns don't know how far you traveled, that's why they won't shut off.