This is VERY common. If the brake rotor isn't cooked too badly, replace Calipers with rebuilts (or new) and new pads. Both sides. Good idea to replace the hoses too. Not as critical as the fronts but change'em once in awhile. The calipers mount on slides and what I call "Keys." Those keys are stainless steel, but the slides and the calipers are iron. They rust and contribute to drag. Clean and lubricate with High Temp Lube for Ceramic Pads. The Keys are a little tricky to remove and install. An OP here, Cloud Driver, has a thread about E-450 brake job that can be a tutorial if you want to fix it yourself. It isn't a difficult job but there's jacking, blocking, wheels off/on and beyond that the Keys are the only tricky part.