The clunking in the diff maybe caused by worn our shims not putting enough preload on the gears making them lose and and creating the clunking sound you describe which I think the most likely cause I would not by another one just pull it out and inspect and then get it shimmed shims from Nissan are only like 13 bucks if ur mechanically minded u could do it yourself ...
If not take it to a shop It should be around the 500 mark to get the work done I think whic is cheaper than a cusco as stated above factory gtr is a 1.5 way anyways ( I think )
As for sensors replace the 2 just after the turbos they are ute o2 sensors the one on the cat is actually a different sort of sensor it is connected to your light on your dash and will measure ur exaust gas temp it will tell ur ecu if it gets to hot meaning it's leaning out to much which indicates more servere problems which is a story for another day
With the hicas Just loop the lines so they run back to the pump ( bypass the hicas when u lock it ) if u buy a proper lock bar kit they should give you parts and info required for the job