Change you shift motor. If your unsure take the shift motor off take it apart I'm sure you'll find that the armature is black. Clean it up with fine sandpaper put it back together and try it. It's not going to cost you anything to try cleaning it up. Putting it back together isn't hard if you put the brushes over the armature first and hold the brush holder tight on the armature and then place it into the case.Hold it good because the armature will want to get sucked into the case by the magnet.
If this fixes your issue then you can buy a new one. There are lots of posts about shift motor problems ,