The nut could very easily require a special spaner socket. The new nut will tighten with a 12point socket but the originals were round with four slots. I have seen many tightened with a chiesel and hammer.
The nut holds the output flange to the output shafts and doing so squeezes down on the output bearing inner race the governor the speedo gear and a few spacers (most of which is inside the trans.
It will require dropping the front of the driveshaft out of the way.
Bosch Master, ASE Master, L1
33 years MB technician