I'd have to agree with JohnMC about the center support bearing. There is a small spring that sits within the rubber mount to help hold the driveline and absorb some of the shock. The metal clicking noise you described is probably the spring coming out of the mount and rubbing on the edge of the driveshaft. If the driveshaft isn't properly supported anymore, you will get a lot of vibration while moving. It only seems to get better when going faster, but the truth is, don't wait on checking this out. This is what happened to me, and since it took me a while to figure out what it was(because it only happens while you are moving), I ended up having to replace one u-joint as well as the center support bearing. I'm actually lucky the front half of my driveshaft stayed attached.
Phil
69 142S