Because of the age of the truck, I think you should just get the pinion yoke nut and try putting it back together. I would think that if it had been destroying itself as this loosened…you would have heard the rear end howling the last few days/weeks.
Have the rear jacket up for room to work and borrow some jack stands to be safe.
First thing to do is grab a can of spray paint and mark one bearing end and that ear of the yoke.
They are balanced and will vibrate if not aligned the same as before. Then slide the tranny end of the shaft back into the tranny. You may have to rotate it until it lines up and slides in.
Note; You could just pull the shaft out the rest of the way and do some of this away from the truck.
Remove the four bolts that hold the retainers on. They will be facing towards the rear and should be 10mm. Pull the retainers off and pry the u-joint out of the yoke, being careful not to let the bearing caps fall off of the u-Joint. This should seperate the yoke from the u-joint. Tape the bearing caps on so you don’t accidently knock one off and you lose the bearings.
Then slide the yoke back onto the shaft…install the new nut and tighten it down. You will probably have to reinstall the retainers and put a piece of pipe through the retainers to keep the shaft from turning as you tighten the nut.
Then remove the tape and carefully…without disturbing the bearings…slide them into the yoke and install the retainers. You may have to have the tranny in Neutral to align the yoke and u-joint again.
From the pic’s…you’ve lost some rear end lube. Any parts store can line you up with the right fluid. Remove the fill plug from the cover pan and refill until the level in the rear end allows the fluid to flow back out the plug hole.
Then check the tranny dip stick in case you lost fluid when the shaft pulled past the seal. Refill if necessary.
Take it for a drive and if the rear end is not making more noise than it always did, you should be good to go.
The one thing I would do if it’s quiet…and not vibrating and you feel confident after driving 50 miles.
I’d make an appointment to have someone replace the u-joint and torque that nut properly.
At that time they can also inspect everything to be sure there is no other damage.
Forgot to mention. If on the test drive it howles and makes a lot of noise all you’ve spent is the price of the nut and your time. Then you can decide weather you want to have it towed to a shop, or salvage yard.