Turns out my problems with the vertical slack with the main shaft was an indication of a VERY serious problem developing from an easily-made mistake!
It appears that during the build, the middle main shaft bearing block was installed with the bearing facing the wrong direction. As a result, instead of riding against the bearing surface, the top of the auto hub sleeve was actually gradually burrowing into the wrong side of the main shaft bearing block. As this wore, it was also allowing the mounting hardware for the main gear and lower jesus bolt collar to move, creating noticeable wear in the auto hub sleeve, jesus bolt collar, and mounting holes in the main shaft! Needless to say, this could have gotten VERY ugly in probably a very short time!!
So, off to the Synergy store for a new main shaft, bearing block & bearing, auto hub sleeve, and the associated hardware for the main shaft. All things considered, this could have turned out to be MUCH more expensive than relatively small amount I'm spending now...