Ahhh the joys of gear diffs
Mines from the second batch and i can allready see the wear on the center driveshaft and the diff outdrives, it's the price you pay for having the direct drive of the gear diff, a ball diff takes a lot of strain off the drive train.
Grease will help, but rather than buy a new driveshaft why not see if you can press the old pin out and fit a new one? You only need some hardened 2.5mm? steel shaft cut to length I know i've got some old hingepins i plan to try it with when mines gone.