I've noticed my front driveshaft spins when I drive even though the transfer case and manual hubs are disengaged. Just curious does anyone know why this happens? Is this normal or does it indicate a problem?

BTW, I'm pretty sure everything is disengaged because I can turn the driveshaft by hand when the truck is stopped & I know the driveshaft is spinning because I get fresh grease splatter around the spline and the U joint...

 

Can sometimes spin due to the drag of the heavy gear oil on the front drive gears. May do it only when the oil is cold (thicker) and stop when it heats up (gets thinner then).