Do you have drum brakes or disk brakes? If you have drum brakes, it could be that the pads are just barely rubbing on the drum, which would explain why you wouldn't hear it while you are braking. Jack up the back, take the rear tires off, and turn the drums by hand and see if you can feel or hear the pads rubbing. If not, check the fluid level in the rear end like said above. I can't really see it being the u-joint, but it wouldn't hurt to check it while your back there.