2003 Trailblazer 6cyl 4WD LS Starts vibrating through the steering wheel and can feel it generally at about 30mph. If I let off the gas, the vibration stops. It is a rust-bucket on the bottom side, but the universal joints "look" OK. I don't feel any looseness, but that doesn't guarantee anything. Any suggestions as to isolating the problem i.e. rear end, transmission, drive shaft?? It shifts O.K. in 2 wheel, 4 wheel high, 4 wheel low.