Look at the u-joints first. If they are worn, it can cause vibration. Simple to drop the rear driveshaft and replace the u-joints (remember to mark the position on the shaft and transfercase output to make reinstalling easy).
As to the front driveshaft spinning, you don't say if it's a slow rotation or actually under power. If it's a slow rotation, it's just caused by the vibration of everything else. If the front driveshaft is engaging, you have other problems.