I'm pretty sure it is NOT your differential if you can do it by pushing
down on the trunk lid. Rear suspension bushings go bad on them ALWAYS
by the time they are 20 years old. I'd also check the rear shock mount
bushings and their attachment to the swingarms. If you've still got the
old style shock absorbers you'll need to get the shock mount update kit
to hook modren shocks to your swingarms. It is an easy fix if you haven't
damaged the holes in the swingarms. I had to have the ones on my 164
welded up because they wore the hole so bad it developed a crack.
--
George Downs, Bartlesville, Oklahoma, Central US