During my last rebuild I discovered the ujoint was rather loose in the transmission tail shaft. The problem was the square hold in the tail shaft was worn. Thinking was the cause of the rough shifts I experienced, I replaced the tail shaft with one with a good fit. It made a very big difference in the smoothness of the shifts from low to high. No jerking or clunking in the slightest. Thought I'd pass my experience along because I've never seen it discussed.
Richard,i believe the tail shafts that were in engines that did an inordinate amount of heavy pulling are the only ones that wore much. A TT that was a much beloved toy actually split the tailshaft.
In the past, I rarely looked at the fit. From now on, I'll check.