It will fail if you put it under great stress .i.e.when off-road. Normal road use just normally makes the thing vibrate like hell before it gives up on you.
They are usually held as a stock item at most LR dealerships and they are around £42 + Vat I think. Paddocks do sell them but I personally would buy the genuine ones from LR as the cheaper ones really don't last as long. Some I've know to go again in 12 months.
Replacement is easy just remember to take off the front end of the rear propshaft 1st otherwise there's not enough room to get the rubber donut off. That said, the front end is really tight anyway and you'll need a prybar to help compress the propshaft to clear it from the bolts on the handbrake drum.
Hope that helps, they do make a difference once they have been changed :D