I got a 1997 GMC Jimmy 4x4 and the transmission lost third and overdrive. It shifts like a champ through second and then just refuses to shift again. I replaced the ignition switch thinking of that known issue but it didn't do anything. How hard is a 4L60E to rebuild?

 

Losing 3rd and 4th can be you lost the 3/4 clutch pack which means you need a rebuild or a new transmission.

Others have rebuilt these transmissions but I haven't.

 

Auto Trans are difficult to rebuild, special parts, procedures etc, dont go there. Maybe an ATI...