I have a 93 Ranger 4.0 5sp 4x4 SC. I have a humming in the drivetrain area. I just replaced all the u-joints and center mount bearing on the rear drive shaft. The humming was a little quieter until I get into 4th gear. The humming though is still there. Any recommendations on what to do next. I am thinking of changing the tranny oil and rear end oil. Any suggestions?