VSS question

06 TBSS took it to the track, made a pass with the scanner hooked up. Mid pass I noticed that the speedometer on the dash, and on the scanner was reading 0 mph. The truck still ran through the gears and made the pass without any issues. Slowing down in the shutdown area, at 78mph, the speedometer came back on, both on the dash and the scanner...what gives? what causes that? is there a fix? Im not worried, like I said, it made the trip with no problems. I was just curious.

Truck AWD and have a looser than stock converter in it? If so that's been a common issue, I don't know if there's ever been a fix for it or even a real explanation for what causes it.