I have a 77 F150/C6/351M/2wd

I've been working on it for a year and have had incredible help from this forum, doing almost 100% of the work myself (still a newb wrench turner).

It has had these problems since I bought it, & not sure if related:

1.) When you put it in Reverse, it shifts HARD.

2.) When driving, the second you let off the accelerator, it downshifts.

What I have looked at/tried to diagnose for the hard shifting into Reverse: Looked for play in U-Joints. None, and it looks like they are new, along with a new driveshaft. Checked rear end: no slop, fluid level is good, and not funky or burnt.

What I have looked at/tried to diagnose for the downshifting: At first, I didn't have the kickdown relocator for the carburetor (PO put on a 4v Edelbrock). I bought it, put it on, and it seemed better, but it might have been wishful thinking. I disconnected the kickdown, and resigned myself to manually shifting it, but I'm not really sure if I did it right (I disconnected kickdown linkage from carb).

What else I know that may or may not have anything to do with these issues: The column shifter is janky. There are no hard/positive detents when selecting gears. It has slipped out of park when on a slight incline. I also realize this thing is almost 40 years old and this could just be wear & tear? The PO swapped out the original steering wheel for an aftermarket job, and I fear he may have gotten sloppy with the job. The neutral safety switch does not work. When starting, in PARK, you have to push the shifter down to get it to crank. If you don't, there's no response from the key. A week ago, I parked on a slight incline, and when I went to leave, the shifter felt different, as if the incline put pressure on the trans, BUT it shifted smooth as butter with no hard hit or clunk.

I'm not sure how to proceed, and really dislike the idea of taking it to a shop. I've searched the forums for ideas, but it always goes to the kickdown relocator, or a worn rear end. This is not my daily driver, but it runs great and purrrrrs down the road. It also recently saved my hide when I had to put the wifes car in the shop for a week (she drove my main vehicle, I drove the truck). This seems to be my last major mechanical hurdle (knock on wood). Any help is appreciated. Since I know everyone likes pics, here's a shot of some new West Coast Jr's I purchased from a fellow FTE'r:

 

my guess would be the vacuum line from the tranny to the manifold

 

Ok lets start with the basics. This trans shifts from 3 things. 1 being vacuum at the modulator. Check that you have manifold vacuum at the trans modulator. if you dont have a vacuum gauge start the truck and remove the vacuum line at the trans. You should have solid vacuum at idel. Make sure the truck is not going to run over you by chocking the wheels "Last thing i need is someone posting they where under there truck with it running and it ran over them" So if you have good vacuum then lets look at the kick down rode. To make this simple make sure its disconected from the carb and go under the truck and move the arm twards the back and use a zip tie or something to make sure its not moving at all. Drive the truck and see if your problem is fixed. My thought is you have the kick down set up backwards and when you release the gas its pushing it back. getting caught in the throtel linkage. As far as your shifter problem goes its very common. you need to look at all the linkage from the shifter to the trans. I bet you find bushings wore out. You can adjust the shifter down below to make up for the sloopy bushings and also adjust the safty neutral switch so it starts in park and neutral only.

 

Oh and the 3 rd thing that makes it shift is the governor but i doubt you have an issue with that