89 745t; shakes when idling in drive, not neutral

[volvocisco]

When idling in Drive, the car can be felt to shake/vibrate through the cabin/steering wheel. It comes in waves to varying intensities every few seconds, the RPMs dip down to about 600 or so. When In Neutral, the RPMs increase to about 900-1000 and the shaking/vibration stops completely.

I tried adjusting the fuel mixture making it leaner, using the knob under the intake manifold. This increased the Neutral Idle RPMs from 700-800 to 900-1000, but the RPMs still dip and the slow pulsating/shaking continues as above when I idle in Drive.

Otherwise the car is running well.

Vitals:
199K mi
just replaced the turbo (used/good w/ lower miles)
rebuilt the head
installed rebuilt fuel injectors
new air filter
new plug wires
new A/C Compressor (A/C not engaged when shaking happens)

I tried swapping the Air Mass Sensor, with no change detected. I have not adjusted the fuel mixture by measuring the exhaust, as recommended in the manual (over my head). Any thoughts on how do this or the rough equivalent most cost effectively?

Any insights greatly appreciated!

Michael