Last year, I had the Muncie four speed in my Camaro rebuilt because it got hung in first gear. All bearings and synchronizers were replaced. Over the last few months I have started to notice a vibration in it. It starts at around 4500 RPM in third gear and continues on when I shift into fourth. Once I get to around 4500 RPM in fourth, it smooths out some, but it doesn't completely go away. The problem seems to be output shaft RPM related. I just had the driveline balanced so it wouldn't be that. Could it be the bushing in the tail housing for the driveline? It was replaced when the tranny was rebuilt. Could it have worn out that quick? Could it be anything else?