engine shudders hard at idle

My 300 TDT has about 30,000 miles on its remanufactured engine. It runs great and gets good fuel economy.
But at idle there is a terrible shudder like vibration. The engine is
not missing, something else is going on. I know there is a bolt with a lock nut on it on the rear of the injection pump. I replaced one about 15 years ago when the idle was a little rough on this engine, and I suspect that this needs adjustment.
Does anyone have experience doing this? How risky is it to adjust that? Is a quarter turn a lot or a little? Do I turn the bolt clockwise?
I am on the road and have to make another 500 miles today. Like I said, the car is running great going down the road, but is going to shake itself apart if I don't get the idle smoothed out, but I don't want to screw things up as it's a long way through Navajoland to Albuquerque from here.
Thanks
Paul
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I was told once that the engine mountings could be worn, giving symptomes like this.

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I know this is going to sound counter intuitive, but I had a problem where my 300SD was running very rough at idle, but not at driving speeds. It finally got so bad I has to keep my foot on the accelerator at lights just to keep it running and to prevent it from shaking apart. After trying almost everything else, I decided to change the fuel filters, which were overdue for replacement anyway. The problem instantly went away.
It doesn't make sense because one would think a clogged fuel filter would show up at highway speeds when the demand for fuel is greatest. However, it may have something to do with the the fuel pump struggling more at low RPMs while at high rpms it can overcome the filter resistance.
Anyway, thought I'd pass it along because if the filters haven't been changed in awhile, it sure can't hurt to change them,
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