inspect the upper motor mount
rubber bushings in motor mount worn
your shift solenoid a is stuck open in your tranny time for a gagage visit
Most likely issue is a motor mount collapsed. The second most likely is a misfire.
What is it?
It would be good if you stated what kind of car and trans you have. It can be either engine or trans mounts especially if it is front wheel drive. Have it checked out. GoodluckJoe
Unless it is caused by pavement or road conditions, the likely causes are:Tires out of balanceTires bouncing due to worn shock absorbersUnevenly worn tires with "cupping" at the edgesWorn out U-joints in the drive shaftLoose lug nuts on wheelsWorn U-joints in the drive shaft become more noticeable the faster the vehicle is traveling, and the vibration will still be present if the transmission is shifted into neutral or the clutch is disengaged, allowing the truck to coast.
Possible bad motor mount
if its a manual transmission in neutral and 4 wheel drive in neutral with brake off. if its an automatic you will have to pull the drive line to rear wheels and place 4 wheel drive in neutral with brake off.
Most vehicles will not allow you to shift from neutral or park into drive unless you have your foot on the brake. It is a pretty simple logic flow in the car's computer.
Could be a frozen clutch pack. Time for a transmission rebuild.
park,reverse,neutral,drive,low2,and low1