This has been documented in the past...some of the early trucks did have a different u-joint setup...the use of of all the same u-joints was, for no known reason, not standardized until somewhere in the middle of 1967...I dont have a part number for the odd ones unfortunately.
That being said, what is your trucks VIN number and/or date of delivery...with that I can give a good estimate if yours might be affected by this...if not, then I would think that the ujoint or ujoints was/were replaced before and one of them was not a match for the stock part...thus allowing you to now return to the factory setup of all being the same!
From what I remember...this os over a decade ago when we have a couple show up with the issue...all 3 sets of u joints were different...the ones on the tcase stub shaft from the trans being the largest, the ones in the tcase to rear axle being medium sized and the ones in the front axle to tcase shaft being the smallest...and by enough that it wasnt as close as what you are showing in your pics...
Lord send your Holy Ghost into our hearts and make the desire of our hearts Your Will.
Pro-choice, that's a LIE, babies don't choose to die!!