when i back up i hear a squeak, sounds like something plastic is rubbing, i have checked the parking brake shoes and the ring around the front axle shafts i cant tell if anything is bad or broken, it only does it in reverse.
i had the same problem beat mt head against the wall trying to figure it out. try spraying some pb blast or wd40 all over the ujoint. it keeps it quiet till you can fix it. fixing it is another story. good luck