I can't tell exactly what part that is, but those look like lines in the casting, which is normal, they do not look like real cracks that have opened up.

 

Isn't that the rotoflex in the rear driveshaft?

 

Yes it is

 

I don't know if cracks like that could cause driveline vibration (my guess is probably not) but given that this is essentially a consumable part I would err on the side of caution and replace it.
I have read on here that the cheap rotoflexes don't last very long and it's best to go with the OEM one.

 

Sorry, I couldn't tell it was rubber, it looked like cast aluminum in the picture.

 

I thought the same thing and wondered for a while why everyone else had a rubber one and mine was metal lol

 

It's certainly on my pm list but was going to bump to the top of the list if it could be causing vibration