knocking / thunking / clatter

[Grant Bessom]

The front end on my 1986 5ktqcs has started to knock, thunk and clatter.  It
especially happens when I travel over slight bumps on the highway.  When I
hit 5k rpms in most gears I get this clattering sound from the trans/drive
shaft area.  The clattering is fast when it happens, very consistent.  I
have to admit that I attempted to drive the car thru 6" of unpacked gravel.
I had to back up and go around.  As I drove down the street, the clattering
started.  I would hear some rocks drop from underneath the car.  I removed
all the rocks that I could find.  I still get the noise.  The car is lowered
and has a three inch exhaust system mounted in the org. area.

Thanks
direct replies are welcome (I'm in digest mode)

Grant