A couple years ago, I had a 1989 Chevrolet S-10 Blazer. The truck would not dependably shift into park. I had at least once when it nearly rolled away, when parked on a hill. Really irritating. Seriously dangerous, too.
I took it to the transmission shop, and they gave me a bucket full of garbage about how the shift linkage was worn out.
The answer turned out to be to crawl under. The shift linkage comes down from above, may have had a pivot point or two. On the drivers side of the transmission is a point where the shift rod connects to the arm that comes out of the transmission. I had to shift down to L-1 to make the connection more visible. Loosen a bolt, and then slide the rod from the shift down, in relation to the arm on the side of the transmission.
Sounds like yours will be the same. The shift rod from the handle has to come down about 1/4 inch in relation to the connecton that comes off the side of the transmission.
Is yours a working email? I'd be happy to draw what I mean, scan it to PDF, and email the picture to you. Or maybe I can find it in a repair book.