I would suggest taking off the shifter cover and seeing if all the shift forks are in good condition, not worn or broken. If you lined up the rails the shifter goes into and it is in gear still, it is in two gears at once, it is in a gear and the shifter fork isn't in it's proper place in the gear, that is the gear is not being moved properly by the fork, or some other such thing. It shouldn't be able to go into two gears at once, there is linkage in the shift rails in the cover to prevent this. This could be worn. It is a small pin, as I remember, that slides back and forth between all the 3 rails. If you have to take the rails out of the cover watch out for springs, pins, and balls. If the forks and such in the cover is all in order put the gears in neutral and put the cover back on, carefully, to make sure the forks all go where they should in the gears. You could drain the oil to see beter the condition of things and to make it easier to see a little when putting the cover back in position. All this isn't all that hard, might help, if you have to take it apart, if you have the repair manual. The AC ones or reprints of AC are the best. Keep us up to date on what you find.