"A" Body 67 Barracuda rear end problem

That is the thrust button that goes through the center of the 2 pinion gear shafts it is what helps set the preload on the rear axles when you adjust the axle end play. Now how the hell it got loose in the rear is another thing you have those two buttons and a pin that holds them together then the ends of the axles press and the larger diameter end of each button so even if the small pin that holds them in alignment is the pinion shafts broke the end of the axle would have to move out away from the button to fall out and it would still be in the center of the posi unit and have to work its way past the spider gears to hit the back of the housing. I think if you look in the side of the carrier were the axle would slide in you will see a hole the the small end of the button came from if you don't see a hole through to the other side a button or part of one is still in the rear and that would mean an extra one was left in the housing and got kicked up by the ring & pinion or the rear was spun with out any axles in it and the buttons just fell out the end of the spider gear which makes the most sense to me. I hope this helps and my vote is for 3:55 rear gears depending on your tire size.