Working on another G35 today and something caught my eye, this is why you cannot replace the u-joints in the driveshaft. Nissan uses some kind of die to stake the u-joint caps permanently into the yoke.

It may be possible to use a dremel with a burr bit to remove the staking so you can press the caps out but I don't know. You would also have to find someone who manufactured a replacement u-joint that would fit. You will have to use a micrometer and measure the existing joint and compare against a manufacturers spec book to find one that would fit. I can practically guarantee no one would be manufacturing/marketing them as specifically for the G35 because they are obviously not designed to be replaced.