Changing rear shock rubbers?

Hi all,
Am conciderring changing my rear shock rubber spacer bit at the top of the shocks in an attempt to stop clunking on rear passenger wheel on pot holes/uneven roads which is getting worse.
Also vibration at speeds 70-80mph.
Am I wasting my time?
Will this achieve nothing and so should I just go for complete shock replacement?
Do new shocks come with these rubber spacers?
Thanks for any advice.
Rgds Ben