2005 mercury mountaineer

I changed rear brakes and rotors but after doing this I noticed drivers side rear wheel was wiggling in and out …when I looked I noticed that the part with an accordion looking rubber boot was really loose this part also has a gear on which I think is part of anti lock brake system..it attaches to the hub also I have to replace it but I don’t know what it is can you help me identify this part is it a axle shaft or cv joint..pleaase help me this is my only transportation and I can’t leave it like this and it is scary to drive it don’t want wheel to fall off …please help…THANK YOU