2002 FORD EXPLORER XLT

I just bought a 2002 Ford Explorer XLT with 4.0L V6 EFI SOHC engine and 51,000 miles on it. Shortly after I start the vehicle and begin to drive the ABS light on the dash lights up. Also, when driving there is a sound that is coming from the back tire( not sure which one or both) similar to the sound of studded tires being driven on bare pavement from inside the vehicle. Took it into a shop and mechanic said could be a hub?? Not sure what means. I did some research online and noticed that some have had the ABS light thing fixed by replacing the rear ABS sensor. Any other suggestions??? Also the hatch back exterior panel right beneath the window is cracked, any suggestions where I might be able to buy a replacement piece or do I have to buy the whole thing??