I just purchased a 2002 s60 AWD. I was backing up, pressed on the brakes and the rear wheel locks up. When I go forward it releases if i put it in reverse and don't press on the brakes it will roll but as I put pressure on the peddle it locks up.

 

Have you had a chance to pull the rear wheels and inspect the brake linings? I would start there. One other thought is that the parking brakes need to be replaced.
My 2002 s60 AWD is notorious for the parking brake sticking if it is parked in wet or damp weather for more than a week.

 

Thanks I will pull the rear wheels this weekend