Where is the usual culprit for a wobbly bedside on a 73-79? I can't seem to find what joints are loose on mine. It leans out about 1" at the rear on drivers side and I have to pull in on it to shut the tailgate.

 

This is a common problem on 73-79's. I just repaired mine and as far as I can tell there are just a number of spot welds that rust and loosen up in the tailgate post/rear crossmember area. My dad "fixed" his with a turnbuckle, some cables, and hooks. A pain when you are loading something. The way I fixed mine was to drop the tailgate and weld some angle iron formed into an L at both rear corners near the tailgate. I also added some triangular shaped gussets to the angle iron. I did use my dad's turnbucke system to hold everything like I wanted it. I kept adjusting it until the rear quarters lined up with the tailgate then welded everything in. When I painted the truck I painted the braces body color and with the tailgate up you can hardly notice them when looking in the bed. And the tailgate posts are rock solid now. The angle iron braces are probably eight inches high and extend out to the seam in the bed floor equal to the wheel wells so they don't interfere with loading a lot. I tried welding some braces underneath in a hidden area but I couldn't get it to stiffen up so I went with the braces inside the bed and all is well now.

 

the culprit will almost certainly be the rearmost cross member. I runs under the tail pan, has 2 brackets for the rear most bed bolts to pass through amd then attaches to the bed side to keep it in place. I have not been able to find a replacedment but plan on fabbing somthing up this winter.