Sean,
If you're talking about these, yes replace asap and change all 4 as they are all the same.
Land Rover Defender Front Hub Rubber Dust Cap - JGS 4x4 Land Rover Parts
What the heck is in the air where you live? I've seen these 20 years old and still doing their job (well just about). You usually loose them long before they perish!
Cheers,
Lou

