I have a 2008 Silverado with the towing option. It is turned on with a button at the end of the shifter lever. My question simply is... does it cause any damage to activate the towing mode while the truck is in motion? Or does the vehicle need to be stationary? The owners manual does not explicitly state this.

Thanks,
dbteeitup

PS: I appreciate all the contributors to this forum. I find it very useful. :)

NO. You cannot damage anything by adjusting the tow/haul switch while in motion. It only changes the shift points and delays it going into OD depending upon load. It may never go into OD which is a good thing to prevent strain on the drivetrain.
It is just a switch telling the computer a different command for driving. It is not actually moving mechanical things such as shift forks and such like 4WD.