You don't have to drop the exhaust and drive shaft to change the flex. It usually requires the removal of transmission mount bracket and exhaust bracket. It's a very simple job.
As for the rear flex, they usually never go out. I've never changed one. One of my cars (that I drive very very hard) went thru three (including original) front flex discs. The rear one still looks new. I did however change middle drive shaft bearing as a precaution.