Is there a mod I can do to turn my ecu from auto to manual. Would like to go from O.D. to C6. Can I also change rear diff to 9", will it affect the E.F.I. I know the speedo will not work.

 

If the truck has an electronic trans(E4OD or 4R70w) then all you need to do is retain the MLPS and permanentely fix it in the D position. If the current trans is an AOD then you dont need to do anything. Changing the rearend will affect the EFI system and reduce performance, the speed sensor input is used for load calculations. Besides that swapping to a 9" is a lateral move at best.. it's really no stronger without upgraded axles, and if you are doing that why not step up to a semi or full floating 10.25.