VERY unusual, hence the lack of responses.
I have an r33 GTST (rb25det) and have had a fair bit of diff back-lash for the last 4yrs. The car has been making over 280rwkw during that time, currently making 320rwkw, the back-lash hasn't gotten worse and no problems with the gearbox either. I also do a bit of motorsports and motorkahna days.
I am aware that the rb25 box has a strong reputation, but a mate is running an rb20 box is his 270rwkw r32 and hasn't had any issues at all.
If you are already thinking of the option to get a rb25 box, then do it, as you'll eliminate two possible issues:
1/ the strength of the rb20 box
2/ the repairer.
Also your driving style can also effect how long you gearbox last
Another issue could be your clutch: I have had two mates blow their gearboxes due to having a solid center clutch. A spring center clutch dampens the shock to the gearbox, during gear changes.