My mechanic told me that I would need a new rear CV joint, but said that it coiuld be months or even years before I need to get it done. If I am hearing a clicking noise from the rear, especially when I back out of my garage, does this mean it needs to get done immmediately. Are there safety issues realted to not fixing a CV joint immediately?

I would get it fixed. Until then I wouldn't go on long trips. it could get worse in a hurry.