If there's play with vehicle still on the ground, get that thing jacked up and find the problem! Something's liable to be flopping around once the weight's off the wheel. Locating the issue is a process of elimination and / or visual detection. Move the wheel, see if the lug bolts are tight. Look at the caliper and spindle in the back, see if they are moving also. Look at various other components and watch for movement. It helps to have someone else shake the wheel while you look. And please make sure there's a suitable support under the car in addition to the jack before doing work or sliding underneath.