rebalancing driveshaft?
 

i replaced the front goetze(flex disc) and center support for the driveshaft in my 85 300d a while back. my memory is a little foggy but i think i remember the drive shaft having splines somewhere In the middle by the center support? or at the back of the trans? and despite my attempts at matchmarking it has been off a spline or two. the car is running well except for the slight vibration and resonance. im sure this is what the problem is as it started right afterward. anyway to correct this without guessing or taking the whole thing out to have it rebalanced?

If you think you simply did not put it back in the same relative position as it was ok running in...
Then , while a lot of work, you could just mark the sections....and carefully move it one slot at a time and re test...
Finding a place which can do a good job of balancing these has been a problem for many people... so I would go with the " Get back to where I started" rather than balancing anew...

Your D/S should be factory marked. The earlier ones were not but in 85 there should be marks. They my be very hard to see but I think they should be there.

At some point Mercedes started marking the shafts when assembled.
I want to say 1983 on, for sure your 1985 has the marks where the
splined shafts come together. Not sure what they look like.
Which means you should be able to separate it and re-connect using the marks.

I just found this searching on "driveshaft alignment":
If it's like the 124, at the base of the splines and the end of the socket there are marks, one has 1 raised ridge, the other two, line 'em up.

Second: leathermang's advice.

Third: I found a driveshaft shop that does truck multi-piece driveshafts.
This is the kind of place to go.

One important thing the shop guy told me as a tip of sorts was that the
Mercedes shaft should have the ends (the triangles) aligned.
He felt that almost any orientation of the triangles being aligned would work.

Check for factory marks for sure. Sometimes, however, you fix one but only to find another bit vibrates... may be you'll need to move on to something else like motor mounts...

If they marked them that would be super... get you a good set of goggles...and a hand wire brush and clean them up...
For those years they were not marked....or if someone has changed stuff in the decades since they left the factory... the FSM is very specific about the 'advantage' of putting paint on them before taking them apart...

When I saw this pic in the Manual and how far apart the factory match marks were I made sure I marked my Shaft well.

The Match Marks were not painted on mine but there are raised portions of the casting where those marks are.

For me the factory marks are too far apart for me to "eye ball" and get them back together correctly.

On the 85 they should be raised portions. Try a straight edge next time...

How does the staright edge get around the Driveshaft Support?

I suppose it is possible use a piece of Cardboard with an area cut out so it fits around the Drive Shaft Support.

In my case I just pealed back the Rubber Sleeve and put a Puch mark on the Long end of the Drive shaft and used a Carbide Scribe to scratch a mark a spline that the Punch Mark Lined up with. (The splined portion of is made of hardened metal; you cannot punch it.)