You've probably got either or both of two problems: Sticking rear calipers that aren't fully releasing, or a sticky parking brake that's not fully releasing. Since the front brakes are apparently ok, you'll not notice a problem with steering. As you found out, the heat can be major; you'll need to check calipers, pads and almost certainly rotors, which may well be warped. The master cylinder and hydraulics are usually split for safety purposes, and I don't know how the split is set up on your car--it might be one front and the opposite rear per circuit, or front on one circuit and rear on the other. If you've got one circuit operating both rear brakes, that's another point at which there could be a problem.
On my 560SL, it was the parking brake linkage; hadn't been used by the PO in heaven knows how long, and was a corroded mess that didn't release fully after I used it. However, there was a godawful squeal underway that told me help was needed pronto. The shop wound up replacing most of the linkage, as the parts were corroded beyond usability; the service brakes were fine and needed no attention. No problems since.
'97 SL500, 40th anniversary edition
'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)
SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)