If you look at the photo you will see, I literally shaved the domed bit off the end of the shaft completely, and maybe another half a mm, AND the thicker smooth shiny collar that the driveshaft seal slips over, needs its leading edge angled/chamfered off to match a contour inside the box, which will become apparent when you look inside in detail.
BUT- put grease on the bar inside the diff, AND the chamfered leading edge roughly 10mm into the gearbox past the seal, and keep grinding it off bit by but until it fits and no grease transfers from the rod inside the diff onto the end of the shaf, or from the chamfered edge, onto the seal face - if you get me??