Just a little Follow up on this. You all were so helpful I figured i'd close the loop.
I got the new stub all put together and installed this last weekend up at the cabin and this time there were a few different findings.
I did check the lock nut on the old one once i got it off... it id not loosen, I see no way in where it could either. its designed pretty well.
1) The first thing I noticed is that the new axel seal wear collar (#29) did not need pressed onto the new stub like my last two installs had needed. I thought this was interesting but continued on as the diameters were still good with no play around the shaft. Maybe just a few thousands off this time around, great no press needed, onward.
2) The install went fine all up until I tightened the final lock nut on the spindle. This is when I noticed the final drive had a few mm (about 3mm) of play up and down the spindle which was also different this time around. I didn't like this and was seeing red flags of my gears moving around on each other, slipping, and also this prone to break spindle wondering out 3mm causing more stress and eventually breakage.
3) I pressed forward and went on with installing it all onto the tractor to see if it would clean up the play once all bolted together. No such luck... once all bolted together the whole spindle with tire mounted had the same amount of play. no good! Frustrating how the same parts in the same order on a different spindle don't fit the same... so much for quality precise manufacturing!
So I ended up getting home and calling Yanmar HQ in GA like a few of you had recommended. Found out my prior installs before I had done it myself were done incorrectly and I was indeed missing an o-ring (#29) and a few shims (#23 and #24) it called for 2 each of the shims per side and I only had 1 each per side. They said to order these reinstall and make sure the final drive gear has no play. Also that I needed to re add the blast to my rear tires to offload the front axels. Heavy implement off the back or not he said this is a must if I was breaking spindles. And like you all had said no 4-wheel drive on hard surfaces and be cognizant of my turning radius especially with a load in the bucket... the wider the better on the spindle.
Lastly, they do have the heavy duty parts still for the spindle! Well at least some of them but not all of them. the parts labeled with a xx-1 (for example 18-1) are all for the new heavy duty axel that is 50mm thick vs 40mm. you'd have to find all the parts on your own as the kits are no longer available, but the parts 18-1 through 30-1 (that have the xx-1 that is would need to be replaced) Myanmar HQ did have some in stock, others they did not and could not get, but he said to ask hoye for the ones that Yanmar did not have as they used to buy up all of the older no longer continued parts. So there is a good chance you could still put together a HD kit for these older tractors! Good to know should another one break on me.
As for now I'm going to rip it all apart again add the o-ring and a few more shims to get rid of the horizontal play of the spindle and final drive, put it all back together and add beat juice to the tires then cross my fingers and hope it doesn't happen again. If it does I will be attempting to find all the HD parts and rebuild with the entire HD kit.
Thanks agin for everyones help... I think we got it licked. ope this helps some people in the future! good luck and happy tractoring!