The original Volvo U-joints could last the life of the car (300K-400K). Especially if you put the cheap scantech U-joints in as "preventative maintenance," ironically you could have U-joints that fail before the original U-joints would have! Again, since the U-joints are pretty easily inspected, and since a bad U-joint will give you a good amount of pretty obvious warning sounds before it leaves you stranded, I see no reason to swap them out on account of preventative maintenance.