I think he wants to remove the nut that holds the 3 bolt drive shaft coupler on the back of the transmission. One version is a hex nut, the other uses a special four pronged gland nut. both have an area that gets peened down for retention.

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1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)