1981 300SD Turbo with the shakes
 

When I first bought her, she had the shakes bad. Items replaced to remedy the shakes:
1. Engine Mounts
2. Engine shocks
3. Air filter bracket & mounts

While replacing the bracket & mounts I learned that I needed a new air filter housing as the drain was not right. A piece of rubber fuel hose came out of the oil separator instead of the metal pipe. So I replaced the air cleaner box with one that has a metal pipe that sits in the drain receptacle tube.

Since the drain has been properly seated, the car gets a case of the shakes every once in a while when I take pressure off of the gas. I also need to get the wheels balanced, but this seems like it is more than those shakes...

Does this happen with a hot and/or cold engine?

The things that when corrected had the most/best effect on curing shaking when engine is hot at idle for me: Installing new injector nozzles/tested/balanced and next was finding and fixing an air leak caused by hard fuel hoses down by the primary filter. (Previously the engine shook at cold and hot idle after Injectors fixed cured 80% of that.) (Fixing the air leak cured about 10% more of the hot idle shake.
On a sticker on the bag mine dampner pin came in is Mercedes 000-074-26-74 and inside of the bag is a paper tag with the Bosch part number# 1 420 560 014 000 (the company that made the rack dampner pin).
It is sort of an accumulation of things. I forgot to mention I also changed the transmission mount (automatic trans). I do not recall if this had any effect or not.
Helped but not as much: new Rack Damper screw and new Motor mounts with 1 new engine shock.
The Rack Damper screw by itself had only slight effect and it was hard to find that "sweet spot" where it did anything.
I changed the Motor Mounts many months after installing the Rack Damper Screw. After they were change I re-adjusted the Rack Damper Screw it was easier to find the "sweet spot" and the effect was greater.
My car still shakes just a little more than I would like. I have not yet checked the IP timing and I am hoping that will also have a (+) effect on reducing idle shaking.

I get a shake related to a high idle from time to time, this goes away with the idle adjust on the dash. But of your talking about the shakes that happen at 60-80 mph, that can be felt in your steering wheel, your steering box may have some play in it causing vibrations like this, could also be in need of a front end alignment.

I'm great at idle since replacing the shocks and mounts. Mostly this is like the engine trying to leap out of the car when I'm slowing down on the freeway. It appears as though it happens when I'm downshifting. Could this be a flex disc or maybe a turbo problem? It really freaked me out last night on the 101...

Should be easy enough to check the flex discs. I doubt the turbo has anything to do with it.

Are there any o-rings that go on the drain from the air cleaner? Anybody know the p/n or specs if there are? I've heard of the turbo-drain grommet causing shakes, what about the Air Cleaner drain?

Out of curiosity, when were the valves adjusted last?

the motor tries to climb out of the bay on decelleration? did you put the lower bolts back into the motor mount? are the bolts still there? sometimes misalignment can make the bolt seem tight, and then fall out when you drive a bit.

O-rings on drains are more of what I think is a rattle.

If you are in motion when you experience this then I think suspension,
alignment, tires out of round or out of balance.

It is really easy to check for out of round, and more common than not.

I doubt turbo.

If it seems smooth under acceleration but shaking when braking or slowing please check suspension parts.

New tires fixed my problem. I also learned to check ALL 4 TIRES when I check my treads...

Thanks for all of your help!

I do need to fix my tie rods. Is there any secret to straightening them before I get an allignment?

I just did my tie rods yesterday. My were bad so I replaced them booth for $80. I don't think you can bend old tire rods back to straight. Atleast I wouldn't recommend it.
My trick was to put two straight sturdy objects on booth ends of the tie rods and adjusted the new to the old length exactly. Tape measure was to hard to work with. Drives straight with less drifting/wheel play. About 1.5 hrs with a pickle fork.