car shaking up and down on 2004 Oldsmobile Bravada

just started today, car seems to have sunk about 3to four inches, tires are fine

2 answers 3 comments
While truning steering wheel sharp left or right?
no,just going down the road
Have tires checked, may have an 'out of round' problem.
are you in any snow
was last of January,but this started after that event