Can you dismantle the air spring ? ie take out the air bag and shock absorber into separate units?
In the days of the Classic RR about 1975 we would remove the shocks drill them near the top ,replace the oil with thick stuff and solder the hole up .

With age comes wisdom
With age comes a pension and the need for economy. Always listen to old people or when they die you will live on in ignorance.
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Learned to 4x4 on 100 mile beach and Frazer Island QLD
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The strut is a two-part item and the spring and damper can be replaced separately if desired (although whether it's worth the effort over just throwing in a new complete item is for the reader to decide). The separate items cost more than the whole unit bought as one IIRC, so it's not a benefit from a cost position. 2012 SDV6 - it's missing a couple of cylinders
2008 TDV8 - it was a labour of love and is much missed