I've noticed that my flex coupling on the drive shaft has some cracks in it. What purpose does this part serve? Is it difficult to replace it? Thanks

 

Sorry... don't mean to state the obvious, but the flex coupler provides some "give" to the driveline. Additionally, it eliminates having a "U" joint on the rear end.

I wouldn't worry about the cracks. The coupler has nylon string embedded in it to give it strength - kind of like adding rebar to concrete. Unless it's out of round, I wouldn't swap it out.

-scott