Hats off to Denny---Denny, your theory of an internal twist in the spline that is inside the yoke seems to have been right on. I "borrowed" a replacement yoke from the transmission repair shop (don't wanna say begged here) and it slid into the transmission tailstock all the way, just as I thought it should. This morning I bought and installed a new front universal joint, using my already shortened driveshaft and the "borrowed" yoke, and the driveshaft went in with no problems or space issues. I tried to look up inside the bad yoke with my trouble light, and confirmed that the spline is cut full depth, so the only answer can be a slight "oink" in the internal spline that is not large enough to be visible but is large enough to prevent the yoke from sliding all the way into the tranny tailstock.