i have had the same prob on my 96 xantia, first thing i would look at is the rear height corrector which is located approx mid rear underside just near the anti sink sphere, there is a linkage from yr high/low lever to this, these have a tendancy to stick/seize with road salt and age the linkage is plastic and can become brittle and break hence the rear dropping and not raising, i would suggest to spray the corrector with wd40 or similar penetrating fluid and leave overnite,try to move the arm manually to un.seize it replace the linkage with new and u may be good to go, as i said this was the case for mine and a relatively inexpensive to fix (about £15) dealer part only. wotever way u intend to tackle this (try to get the back end up on some ramps if poss) be extremely careful chock the front wheels securely. hope this info helps good luck.
cliff
January 2010