If the car is jacked up with no weight, the axles at maxed angles and full reach will "vibrate" a little anyway. Check your motor and trans mounts out, see if you broke one. Inspect the rubber in all of them. Could be a bent axle too, have you been off roading lately?
If you get the car up to say, 70mph, and toss it in neutral, does it still vibrate or does it go away?