This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
Many late 60's and early 70's A-body Chrysler products had a problem
with the rear mount for the torsion bar. Water collects in the channel
and rust occurs. After a decade or so the channel that the mount is
welded into rusts through and the mount twists and that side of the
car falls onto the rebounce (sp?) bumper. If this is what happened you
will need to find a local frame/suspension/alignment shop that has
someone who has welded in new material to replace the rusted stuff and
then realign the ride height when done. (Thanks, Chris Jardine).