1998 Cadillac Deville 98 Deville wobbling like driving on i

my grandmother has a 98 deville and has had it for quite some time now. I drove it to school one day and noticed that the car was wobbling from side to side feels more like driving on ice. I want to say it feels like its coming from the rear. Its not a daily driver but just worried for her safety. Has new tires. What could be her problem?

Thanks,

Your help is greatly appreciated