I would normally concur with the other posters here but on this
particular vehicle I would also suspect the motormounts, which are prone
to failure because of the way people have to have give them so much
throttle (damn near floor it) just to get them to move.
My TBI 3.9 was such a dog you had to "drive it like you're mad at it."
The worn-out motormount will kind of sounds like you're going down a
gravel road although you're on smooth pavement.
If that's what it sounds like, check the mounts.
Otherwise, note whether it makes the noise under acceleration only, or
all the time, etc. Depending on that, you may try going down the street
about 20-25 and killing the engine (push in clutch if equipped) and
seeing if that changes it (or makes it disappear, indicating a bad
motormount)