a bad axle could vibrate in neutral if its really shot, but i think you would feel that part being bad all the time at this point, like all speeds.
maybe a warped brake rotor? how the brakes feel? they smooth, or pulse under your foot?
or maybe a wheel bearing. jack the wheel up, grab the top of the tire, and firmly push and pull the tire, if it moves the hub is worn.
while its in the air, grab the tire at the 3 and 9 o clock, and push and pull the tire, it should not move much at all. if it moves, you need tie rods.
