I remember this mentioned on another forum and someone suggested that the slip/splines on the rear section of the rear driveshaft (where the black boot is) were NOT greased from the factory. Maybe have them check that?
Mine does it too. (Did it stock and lifted, just the same.) If I were to take it in, I'm sure they'd blame the lift for the ungreased-from-the-factory slip, so I'm thinking of just doing it myself.