My truck will not start in park or reverse, the lights on the dash will get brighter but nothing happens with the starter

Will not start in park or reverse when turning the key but the lights on the dash will get brighter and nothing happens with the starter. When I put it in park or neutral and jump under it and jump the starter it will start. I have replaced starter, ignition switch, key ignition cylinder, ignition run fuse, starter relay, an I am about to replace the (nss) neutral safety switch if that is not the problem what possibly can be?