That end plug looks like it’s almost fully unthreaded itself - should be seated under flush on that bore and have a kotter pin through the slot to lock it, but guessing it was missing.
Glad it wasn’t catastrophic!
Bingo. When you touch every part of the truck you know what's wrong and usually how to fix it.
I agree with @toy_tek, that end plug is missing the cotter pin and worked itself out via driving. If those are 555 rod ends, they come tightened...