its corroded and has started resonating. are they replaceable ? are they needed ? can i just rip it off ? (my99, btw)

 

The heatshield is held by 6 or 8 clamping setscrews, retained in nuts spot welded to the shield. If they are badly corroded you may be unable to get a socket on them. You can do it quite easily with a chisel, hacksaw or burn them off.
You could refit second hand heat shields or use exhaust wrap from Demon Tweeks or similar. Keeping the exhaust hot results in better gas flow but probably of more importance is keeping temperatures down in the engine bay and in the viscinity of the header.