The 2 piece shifter cable that runs from column back down threw a metal spiral bracket under carpet, threw grommet then under the truck to the second cable can be tightened at said connection. Dont know if that will help but there is a problem from where that cable hooks under dash down to where it ends on transmission. If it's to loose it wont make all the positions. Too tight it'll take a lot of force to shift in gear. Changing cable isnt bad other than that spiral bracket that cable intertwines up through to keep from rubbing against steering column I assume. It can be an uncomfortable position to get at the bolt that holds said spiral bracket in place but not even certain f bodys or g bodys have this bracket like the trucks with the 60E but Im pretty sure they have this type of bracket. Anyway, I know this is an old thread but hopefully this helps someone else searching for answers about this problem. I'd start with that cable though if only problem is not being able to manually shift to first and in drive it shifts fine in first to second and back to first. Cable is in a bind somewhere and is not a transmission problem if that's the only issue.