Sounds like its time for a whole new drive shaft my friend. I have been through similar issues myself. I hear my D/S slap my heat shield every time I miss a perfect clutch(if the revs aren't matched when I let out) and I have this wonderful vibration if the car is in gear but coasting in any gear. I tried the guibo first, then I put on a center support and it still makes noise. I have a bad u-joint, which is non servicable at home. I am just going to buy a whole new one and get a new set of half shafts while I am at it, my boots are totally gone.