It COULD be the auto transmission, but isn't too likely.
Check to see if it does it in all gears or just one.
Most likely the issue is a flex disc, u-joints or center bearing. They are all related.
Check the connection of the driveshaft to the transmission and check for cracks, hardness and / or strings hanging from the joint.
The piece should be completely free of cracks and somewhat flexible.
Driveshaft components tend to vibrate at around 2500rpm in any gear.
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