I guess it could be the frontmost tranny crossmember but its also the engine mount crossmember. Its under the joint where the engine mates with the tranny.
There a relatively square jacking plate on the bottom of the engine/tranny crossmember. This piece in question is sandwiched between that jacking plate and the bottom of the crossmember. Its right below the front swaybar and steering rack. I cant really see how its dampening vibration as the jacking plate is solid steel and spanning the same exact space but I dont know.