You need to take the pin out and remove the lever, there is probably a deep grove wore in it where it rubs on the detent plate.
If you have or know some one with MIG welder they/you can fill in the worn area and grind smooth, or you can locate a new shift lever.
Sometimes the pin is hard to remove, there is probably a tool for doing it but I just use a cheap C-clamp and drill a hole in it large enough for the pin to go through and tighten a small short pin against the pin that is in the shift lever pushing it through the hole in the C-clamp.