I'll bet you the little plastic spur gear inside the drive unit is broken in half or missing teeth. Ive seen this happen alot and depending on how the gear is oriented, sometimes it will grab the mating gear and sometimes it will just let the motor spin.
The real problem is that plastic gear itself, it should be metal.
You can buy the upgraded metal components here, its the 1st item. If you have power folding mirrors, this is a smart upgrade because even if you dont power fold them much, all it takes is 1 manual fold cycle from the guy at the car wash and that cheap plastic gear usually goes snap. Once that plastic gear in there breaks, the mirror main spring doesnt get compressed and it lets the whole mirror wobble and vibrate as you are going down the road.
http://www.gruvenparts.com/shop-by-car/gm-truck