Transmission shift cable fell off side of trans

The arm on the transmission is supposed to have a little round ball on the end of the stud that the cable snaps onto. It appears that that ball has rusted and snapped off. You're going to need a new arm for the transmission. Luckily, this is something that can be pulled off a junkyard transmission in under 5 minutes and only cost a couple dollars. It bolts onto the transmission shifter end on the side of the transmission case with a single nut. When you reinstall the cable on the new arm, you may or may not have to readjust the cable. Probably not since it's the same cable and transmission, but just something to keep in mind. And even if you do, it's a pretty simple procedure to adjust.

I'm glad to hear that even new from the dealer the part was under 10 dollars. As for the technology and design behind it, GM and other car and truck makers have been using that arm assembly style for column shifters for well over 20 years without issue. It's a little confusing how the arm goes when you're assembling a transmission from scratch and took it apart months ago, but it's not that bad.

I'm not sure if your cable is the same as the one on my uncle's dually, but it probably is. There should also be retainer on the cable end to lock it on. If you do replace the cable, it is a 1-piece that goes into the cab under the carpet, up the column, and to another ball joint under the steering wheel. With the carpet and seats out, it's not that bad to replace. With the carpet and seats in, I can imagine it'd be a bear to get done.

For a quick fix, you could probably zip-tie or bailing wire the cable and arm together and it would hold fine. There's not much stress on it other than forward and back motion.