The front U' joint yoke has a bolt that holds the yoke on. This also pinches a spacer, speedo drive gear, and a spacer pipe between the rear trans bearing and the yoke. The gear is free floating unless pinched between the two spacers.
All this to say the bolt is loose so the drive gear is spinning on the shaft.
Here is a breakdown of a car transmission but the truck is the same.
Link to parts book illustration (see 4.345 spacer, 4.337 gear, and 5.579 spacer/pipe).
You will need to split the U' joint to tighten the bolt.
One thing to watch for is, if the bolt has been loose for a long time the spacer pipe will wear on it's ends. When this happens tightening the bolt will not be enough to pinch down on the spacers and gear. Instead the washer in the center of the yoke will bottom out on the end of the shaft.
