When you say "..writing to shaft, like the originals.." IF you see any writing that says
"VORNE" that side mounts to the front. There is a chance someone got one of the old ones backward. Other than that, uuhhh... what everybody else said.

For the nut, big crescent or one like it, fish scale, little thread lock on the threads, helper or two to hold the shaft... I helped a friend with his like that a couple years ago - so far so good.

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1987 300D (230,000 mi on a #14 head-watching the temp gauge and keeping the ghost in the machine)
Raleigh NC - Home of deep fried sushi!

For the first three days after I replaced the disks, I would get the vibration just as described, exactly the same every morning, until things warmed up.

This morning I backed out of the garage and headed down the road. This time there was NO vibration.

So, either it was sitting in the "right" position overnight and will vibrate tomorrow, or it has healed.

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2012 E350
2006 Callaway SC560

You have to leave the center bearing bracket mounting bolts, rear trans mount bolt, and drive shaft slide nuts loose and push the car foward/backwards a few times for the drive assem to center/align itself..
...then you tighten everything up.

Sorry to hear about your vibration, mine was the center support. If ya spent the money on the flex, change the center support. It's only 30.00 and the labor. Read my write up it will help.

Where is the write up? DIY?

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red

Bad motor mounts cause the car to clunk when your dirve it.....

I would look at the other parts of the drive shaft including mounts, bearings. Have you looked at your differential?

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)

Hi Brewtoo,
I had a really bad vibration on my '75 280 that was most noticeable around 35mph, and it turned out to be the plastic bushing that the driveshaft front centering pin goes into, and that keeps the driveshaft centered - it was worn out and didn't keep the driveshaft aligned. Replacing it wasn't too hard, and it was very cheap to buy. (The driveshaft sort of "plugs" into the rear of the transmission as it fits up to the flex disk, and the plastic bushing is what holds the shaft centered - the one on my '75 was goose-egged from wear)

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C