I am having a problem with my 2002 F150. Sometimes when I back up, the rear wheels lock up. It first happened several years ago and seemed to happen only when I had the truck heavily loaded (topsoil or gravel). Lately it has been happening more often and with no load in the bed. It also seems to happen more when the truck is first started in the morning. I am not sure where to look. Parking brake? ABS?