Do a search on key words like wagon, shocks, level and read up on diagnostics. The problem could be with the pump, lines, shocks and/or level sensor. If it turns out you have bad shocks, you don't have much of s selection since the shocks are also the hydraulic actuators.

Lot's of useful bits in this thread:
http://www.peachparts.com/shopforum/showthread.php?threadid=23971&highlight=wagon+rear

Sixto
91 300SE
81 300SD