Casey,
What you describe sounds like the front pin used on 3/4 ton and bigger trucks. The small nut is on the end of a tapered lock pin that holds the shackle pin in.
Loosen the small nut a little and then tap on the nut to loosen the lock pin. Finish removing the nut and drive the lock pin out.
From the inside of the frame, you should then be able to use a long drift punch to drive the shackle pin out.
The shackle pin itself is smooth like a kingpin with groove in one end for the lock bolt.
