Connection points on the drive shaft. There are two, one at the output of the transmission and one at the input of the differential. Right before they fail, they make a loud clunk sound when torque is applied to them.
Its not that bad, they are pretty easy to get to and replace. Remove the driveshaft and press them out, put new ones in and replace the driveshaft. There are instructions on how to do it on the web, even videos on youtube.