Wheel Bearings
Drive Shaft
Loose Wheel(s)
Or the really bad one- Torque-convertor
Jack one side (front), UNTIL WHEEL IS OFF THE GROUND. Jerk the top of the tire, if you hear/feel movement, loose or bad wheel bearing. Check both sides.
Wheels chocked place transmission in "N", rock back and forth to release any slack. Get under there and see if the drive shaft moves. Up and down/side to side. Start at front working your way back to the swing bearing and on to the rear. Check pinon on rear end to see if you have movement there.
Or, spend 1hour labor charge and have a shop check it out.