You replace them. Once they leak they are shot. Just stick w/ stock mounts as replacements as poly mounts will introduce vibrations again.

And yes, get them replaced. If the mounts are worn/bad they will cause your engine/trans. to sit at a different angle which can put extra stress on the cv axles thus causing them to wear out quickly as well. It changes the angle at which they sit (kinda like the pinion angle on the RWD cars).