rear wheel alignment?
 

1998 Deville has slight rear side to side motion as it drives down the road. On snow it fishtails back and forth dangerously at 30 mph. Car has 90,000 miles. Problem for 40,000 miles and two sets of tires, even after rotations.

4 Wheel alignment is worth doing if rear toe-in is way out it could cause "fishtailing". Also check both front and rear suspension for worn,loose, or broken parts.