Assuming you have checked the mounts on the rear engine cradle and the front
transmission mount AND they all check out OK, the other possibility is the
rear spring plate bushings.  Shaped like a doughnut, these mount between the
spring plate and the rear torsion bar.  They could be possibly worn so much
from movement that the spring plate itself is contacting the torsion bar
housing and scraping it when the plate is moving.  Sounds horrible and can
make you think the whole rear is ready to fall out sometimes.

Jim / BusBoys@aol.com