From my years as a certified Ford mechanic in Queens NY I can say with some degree of certainty that the noise can be aleviated by simply securing the dead body in the trunk so that it does not roll around. Also it could be a bad U-joint.
Check you E-Stop Brake on the rear.
If your truck is 2wd than check the bearing, hub assembly. This is considering that you have the balance and alignment checked at a shop already
remove the plastic milk jugs and discard them. The clunking noise should stop.
If there is a clunking noise on a 1991 Firebird Formula 5.0 when going below 30 mph, and the clunking noise does not happen when the clutch is depressed, it is probably a problem with the flywheel. The flywheel moves freely when the clutch is depressed. When the clutch is not depressed, it could be that something is rubbing on the flywheel, like the clutch plate.
Sounds like your ball joints are shot. Could also be your axles. I had the same noise, clunking when i turned and it turned out to be my axles. Check the boots to see if they are ripped.
chiken
probably your rearend
tie rod
You need struts
Bad shocks
Probably something is loose or worn... sorry to not be more specific, but you'll need to track it down to see WHAT is making that noise.
It COULD be a constant velocity joint (CV joint) going bad.