scottydogs is correct about the linkage; it is controlled by a "back-lock" linkage that is driven by the shifter.
I've got the 1969 Factory Service Manual pages for the back-lock linkage adjustment procedure. I believe the adjustment procedure is the same through 1973 (maybe all the way to 1977 on GP) I tried to attach the scans but the files are too large. If you PM me your email, I will send the scanned pages to you.
Make sure the bushing at the column-to-shifter rod linkage is present and in good condition. It's a cheap little part that causes all kinds of aggravation when it is worn or missing.
