You got it right about that nut. It's for the spline, once you loosen it, the shaft should slide into the splines and make the room you'll need to re-install.
Spray it with some Liq wrench and let sit for a while (if/when you get the shaft loose, mark the front and rear shaft so you can re-assy the same way, pull the front shaft off and lube the splines of the rear with synth grease and run some grease in the end of the front shaft, this will affect easier dis-assy a few years from now.)
If you just can't get that nut loose, you can jack up the rear end, support the subframe and then un-bolt the diff mount from the car (two bolts) and let the diff drop. If this doesn't give enough clearance (worked for me), then you can also un-bolt the rear shaft (easier to pry on, get to, etc.), hook up the front with the rear dangling and the center mount D/C'd. Once the front is bolted together, re-mount the center support, then work on the rear. With the diff low, you "should" be able to pry the disk back together.
If I were close, I'd come over and do it for you.
Good luck. I'l be cussing right along with you, have to remove my shaft again too!
1984 300D Turbo - 4-speed manual conversion, mid-level resto
1983 300D - parts car
1979 300TD Auto - Parts car.
1985 300D Auto - Wrecked/Parts.
"If you don't know where you are going, any road will get you there". Lewis Carrol