2004 Why xj8 suspension

They may or may not all go out at once but to preserve the handling and safety it would be best to replace all of them at once,

Ditto, replace them all as a set, not just individually. (I would only replace an individual one if all of them were fairly new, and one took damage, then I'd replace just that one.)

If the compressor fails for any one of several reasons, all struts will slowly deflate. This system is a fully controllable "suspension adaptor system" and each strut is controlled individually. This controls brake diving, acceleration lunging and leaning in turns. With mine the accumulator pressure sensor failed and all struts slowly bled down.If the compressor is functional it is possible to turn it on to pressurize the accumulator for and opperate for a while!! There is a relay in the fuse box nearby for the compressor. By jumping the front and rear terminals the compressor will run. My accuulator and pressure sensor are under te spare tire.

HEY Since each strut operates completely independent I would only replace the faulty one.