You don't have to go to either of those expenses jacabus.
Naturally there will be flex when you're turning which may have contributed to the wear and will also cause you to hear the rattle more often if you've gone round a bend. The heavy tailgate is supported by 2 brackets at the top, but at the bottom there is only the lock/striker keeping it in place.
Opening and closing of the tailgate will also have contributed to the wear.
*I haven't seen this mentioned on here, so perhaps people aren't aware of it. The wear and subsequent sideways movement causing a rattle on the K12 tailgate is a known issue by Nissan and they have issued an official 'fix' to dealers for it.
One way to fix the issue would indeed be to replace parts, but Nissan have issued a fix which avoids having to replace parts.
The official fix will only cost you pence and perhaps as little as 5 minutes of your time. Believe it or not all the official fix involves is wrapping electrical tape or such like round the hook part of the striker that the lock closes on to and that is it!
The Fix - All you will need is some electrical tape or similar and some scissors. What you do is wrap the tape tightly round the front facing edge of the hook part of the striker. This has the effect of increasing the circumference of the hook that the lock closes on to and thus eliminating the sideways movement and hence the rattle. This may involve some trial and error - putting some tape on, test driving, then adding some more tape if required or removing some tape if it becomes particularly difficult to open and close the boot.
I can confirm that this fix does indeed work. I was having a complete nightmare trying to fix my boot rattle. The rattle was so annoying and it was so frustrating not being able to cure it. I tried everything you tried and more. I had everything stripped away from the boot on several occasions and at one point had somebody lying head first in the boot with the rear seats flat, as I drove around, so I could definitely be sure I knew where the rattle was coming from. It was indeed the boot lock and no matter what I tried, after many hours of trying and many weeks of trying different things I just couldn't cure it, and I couldn't understand how there was still a rattle, as the tailgate was shut extremely tightly? Then thankfully I found out about this fix and the lateral movement/wear!
I did mine several months ago now and I'm still rattle free. I don't know how long the fix will last for, but for me even if it did mean replacing the tape once or even twice a year then that is worth it over having to go to the expense of replacing parts. Some people may not like the idea of using tape and would rather replace parts, but like I say, it is an official fix from Nissan and mine has stayed rattle free for several months now. I think it was November when I did mine.
The only slight negative of this fix was that the boot stuck a bit when opening it, but perhaps I used more tape than I needed and anyway it is much better now and for me a slightly sticky boot is much, much more preferable than having a near constant, loud rattle.
If this is your issue and if it fixes it, then great. Then if not, all you have wasted is a little bit of time and some tape.
I can upload some pictures if you like, but it really is all very simple.
I hope this helps you