At higher speeds (50+ mph) the rear end tends to kick to the right whenever I hit a bumby road. You feel the rear end jumps sideways and have to adjust the steering a bit to compensate.
Rear end jumps sideways on a bumby road. on 2005 Buick LeSabre
Have suspension parts checked for looseness and wear. Also check tire air pressure to make sure they are not over the reccommended air pressure. You can do a bounce test to see if shocks are worn or seized. I put my knee om bumper and push down with all my wheight at each corner of car to feel resistance differances going down and then let off the weight to see when body comes back up if it appears to continue to bounce up and down when I let off to indicate a worn shock. Your car may have a seized up shock and it would be indicated by being hard to push down on one of the rear corners.