'98 Sonoma vibrating when accelerating

I have a '98 Sonoma with the 4.3 V6. It has 143k miles on it, and I have never had any problems with it. In the past couiple weeks though, I heard a tapping noise like metal against metal when accelerating from a standstill. Now there's not just the tapping, but the whole truck vibrates and shakes whenever I accelerate even at high speeds. The tapping and the vibrating sound like they're coming from the bed right behind the driver's seat. Has anyone ever had this happen before or have any idea what's going on?