[ChevLoRay]

A couple of things come to mind: (1) Under the hood, on the column, look for the rod that goes to the transmission. Where it is attached to the column, it goes through a "keeper" that has a compression fit. There is a screw on it that has to be tight, to keep the shifter from doing nothing. Had one that worked loose. Truck was in gear, and the shifter couldn't change it. Screw had loosened up. Placed truck in neutral (manually) and aligned the indicator to show neutral, then tightened it up (you'll need someone to help).
(2) At the transmission, there is a part that is U-shaped (allow a little leeway with the description). It is moved when you select a gear position. It then works an iron piece. The piece that is made from iron will eventually wear into the U-shaped piece, making gear selection vague.The GM part number is: 3904770. It is actually called a shaft. The group number is: 4.040. In the past, people at transmission shops have told me that you can't get them anymore and you can braze them the worn places to tighten up the shifter. But, check with your local dealer and give 'em that part number to see if it is still good. I replaced mine, several years ago, and I have a spare. They may be different for TH350's and TH400's....I don't know for sure. I have a TH400 in my truck.