you could start by opening that
shift linkage box on the column and cleaning/re-greasing it - have some one shift from inside and watch the way it works, it may need some adjustments so the forks in there are properly lined up in neutral, the fittings outside it can be adjusted .... check for wear of the fork corners, and make sure the spring inside isn't broken, and check the bushings on both ends of the rods from the box to the trans for wear/slop
Bill
