As above, if the work done recently checks out, the likely cause would be the centre bearing.
With the old motor mounts, the drivetrain would have "sagged" out of the OEM position, and the centre u-joint would have worn in in that location. The u-joint doesn't change angle very much, so it will take a set. New motor mounts would change the angle, and could make it grumble.

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1984 300 D runs, interior
1984 300 D light hit in front, RUST, mint/perfect dash, decent interior (see above), parts car
(hate to kill one)
1998 Dodge diesel