About as easy as anything on an XM! Look along the anti roll bar and you will find a clamp bolted around it, secured with a single bolt. Connected to this clamp is the linkage that transfers the movement/position of the suspension to the height corrector. Spray the linkage with WD40, two or three times over a few days and it should free up. Once it has apply some grease, which should stop it siezing again.
Depending on the engine and year, you may have to remove a heat shield from above the transverse rear silencer. It's held by two small bolts that screw into the transverse crossmember at the bottom and by clips at the top. The bolts usually turn forever (use a big screwdriver between the shield and the crossmeber to apply pressure and stop the nuts rotating). The clips usually rot away after a number of years.