I may be way off but on my 83 the spring thingy that keeps the front brake pads from rattleing are gone and I get a similar noise (but no vibration) that sounds like it is the driveshaft, kind of like the "tink" of popcorn in a metal pot. It started real faint and as the pads wore got progressivly worse, I was under the truck doing the same thing you are doing until I had a buddy drive it up and down my street while I walked beside it. You may want to try that.
