First, inspect the Nivomats for leakage at the bottom of the piston. To test:

measure the distance from the ground to the bumper. Place a very heavy weight in the rear, like your neighbor holding a 12 pack of beer? Push down several times letting the back of the wagon pump up and down. Then measure again. The distance from the ground should be close to the original measurement.

Or drive around the block with the weight in the back. That will also 'pump' up the Nivomats.

On my 1998, I had one die. Easy to replace, but the cost was a LOT. I wound up at a junk yard and bought a pair.

If your car rattles a lot from the rear, consider changing out anything that has a rubber bushing.

Front shocks, I went with Koni Sport $$$, but original springs. HD parts.
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Keeping it running is better than buying new