I stuck a gear vendors O.D. on the back of my sm420 and couldn't be more pleased. My rear is 3:90 w posi. I drove to Portland and back recently and averaged 75mph on the way back. 4th over is the equivilant of having 3:08 rear. It's an expensive way to go, but it gives you the best of both worlds. Also, 3rd over is AWESOME for hills and in city driving over 30mph. This way you also get to keep your granny and use the truck the way it was intended.
