Car shakes when in gear?

I suspect that your car is moving down the road, If so - it could be anything from a bent/out of balance drive shaft, a bearing in the transmission, any of the bearings in the drive axle assemblies, a very warped rotor,a way out of balanced tire, a bent wheel, a tire with a cord shift. Another thing it could be a front steering problem-such as a loose/worn(a lot) steering component-ball joint,tie rod end,or a idler arm. Also loose lug bolts/nuts, or worn/loose wheel bearings.

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If your car is not moving... you may also check the motor mounts.