The "bottle cap" you see is a continuation of the drive splines on the pinion. The "hog ring" retains the yoke on the shaft. It looks like rust retains the yoke now! You need to pull the yoke off the shaft, it is ready to come off after the snap ring has been removed. A quality puller and some rust remover and/or heat will be needed. Hopefully you have not destroyed the end of the shaft.

What he said. Needs driven out with a press or BFH.

Take Andy's advice. He knows these cars for sure! Sorry if I misguided you. I just have never seen such a thing. I miss the days of drive shafts and U-joints!

What Toots said.

Bottle cap enigma
 

Hello: A truly amazing story. I am the lowest grade of backyard mechanic and am very interested to see what your solution is. I like the "looks like splines" comment and bet it takes heat and pulling to remove it.
Let us know and much respect and many thanks for your persistence.
Riviera8

Just my two cents, Andy's right, that "cap" is not a separate piece but is part of the shaft. During manufacturing the splines are machined to the end of the shaft, then the groove for the retaining ring is cut in. Those splines need to still be in good shape if you want to be able to press that yoke off without damaging it. Hopefully they haven't been boogered up too badly from the vice grips and screwdriver. If they are, either scrap it and look for a replacement or try using a dremel or file to restore the splines on the "cap" so that the yoke can slip off.

I just sat down to see what is going on and carefully read through this thread. I agree with the assumption that the bottle cap is nothing more than them the extension of a threaded shaft. A clip was placed in the groove to hold the yoke in place. I suspect a better job of cleaning off the YUK and RUST might have shown it had it been done.
Like Ruby, a lot of time has gone by and I probably question whether I have seen things like that or not but then again I don't need something with all wheel or four wheel drive and probably would not have bought one 'specially with that kind of mileage on it. But, and there is always a butt, I have a 12 ton press and used it to change a front wheel bearing one time when (actually twice now) the service center for Saturn (hey still had dealers) said it was a non-serviced item and could not be replaced. But they had the parts so I bought them. Good luck on your project. I have to admire some one who tackles the improbable things and succeeds.

They just didn't want to do the work on pressing out the bearings without trashing the hub...my Saturn Tech spent almost 45 min replacing a wheel bearing on my 95 sc2 so that the hub was not damaged.

And I too admire those who conquer what others deem undoable. Takes a certain drive and mindset to do such things.