As you know, it's supposed to drop maybe 3-400 rpms when in D, this is about what mien does. And mien shakes bit too w/ no throttle on at all, but not bad.
It sounds like you have (as mentioned) perhaps a small vacuum leak, or loose/worn mounts (guesses). You'd get the same effect if you put it in park and used the idle screw to idle it down to 500 or whatever.
What is the idle reading in D?

With 200+ Billion electrical parts, the world most complicated machine is inside your own skull.

Question Reality.
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'89 Rnr DLX "SR4.5", 32s w/ 5.29 locked f/r blah blah