Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
Check the u-joints on the driveshaft and while you are down there look to see if there any clean spots on the driveshaft where weights may have been and flew off. If so, remove the driveshaft and take it to a driveshaft shop and have them balance it. To check the u-joints, you can use a screwdriver or prybar and stick it in the u-joint and pry and look for movement. usually though when they go bad you hear and clang when shifting from reverse to drive or drive to reverse.
If the driveshaft checks out or even before that, if you haven't had the tires balanced, I would do that.