Curt, you beat me to it.
Some of the confusion may be due to differences between the Chevy and GMC trucks. I cannot comment on the GMC, but Curt is dead on. The Chevy AD 3/4 tons had a three speed on the column as standard with the four speed a RPO (Regular Production Option).
On the driveline, if you don't believe us that the front was enclosed on the AD Chevy 3/4 tons, go read the Shop Manual, specifically page 4-52.
