1995 International 4700 Rear Brakes Dragging

I've got a 95 4700 single axle with hydraulic brakes and the rear brakes are dragging. It's a flatbed truck and we are moving round bales about 6 miles round trip. The last time I hauled a load after I got back with the bales the right rear rotor was red hot. The truck isn't used all that much and my uncle had taken it about 35 miles a few months ago and the left rear rotor was red hot and he parked the truck came home and we went back a day later and drove it back no problem. Does this sound like a brake line issue? I would assume we want to replace the rotors, pads, calipers, and maybe the axle seals since it got so hot.

 

Hulld, thanks for the reply both calipers had seized pistons. I also threw a set of wheel seals and a pinion seal on while I was at it.