I've got the same thing happening on my 88 auto.

Mine turned out to be the exhaust. The support bracket just behind the catalytic convertor rusted loose allowing the catalytic convertor come in contact with the middle crossmember.
It's most noticable at lower RPM's like drive or reverse as opposed to the higher RPM's of neutral or park.

Hope it's that simple in your case.

dT


'89 Monty LWB auto, 3rd row seating, 95SR rear + coils, genII UCA's and calipers/rotors, dual bouncies.