Daniel;
The top of your transmission has all the shift links in it. there is something loose or worn in there. You can get at it by removing a floor panel and knocking the top inspection cover (with the shifter) off. You'll need your manual or the service of a tranny shop to align everything before reassembly.
Keith;
I changed my diff ratio from the original 5.14 to 4.10 from a '72 3/4-ton, and it changed the whole character of the truck. I use low in my SM420 to get up a boat ramp or to climb a sidewalk, and to get a big load moving up a grade. Otherwise, I still use only the top 3 gears, but now I can run 55-60 mph all day, which was out of the question before. The engine is still the original 216.
