I noticed a small crack in the rear flex coupling on my02 Disco. It's not very big and I dont feel any vibration is this a "must do" ? or is it common for them to crack a little like on BMW's where they call then "guibos"

What's up with replaceing them with a "u joint"

Is it a hard job? It doesnt look like it.

Any insights?

 

The conversion is very easy. I did it even though my flex joint was in good shape.

I guess the size of the crack and the overall condition of the material is subjective. If you're not feeling anything, there's probably a good bit of time left on the original. I don't think it's going to fail catastrophically on you.