Good advice from diyer, he lives in some real mountains so if anyone should know, he would. Main advantage of disc brakes over drums is lack of fade, so towing a trailer without brakes down grades I could see a possible issue with your rears in that instance, but unloaded around town something else maybe going on as others have mentioned. In the meantime before any brake upgrades, while towing down a grade, make use of engine braking, use the tow/haul mode and manually downshift to a lower gear to let your engine help your brakes out.