I know this shows my lack of knowledge when it comes to rear ends but is it as easy as switching out the third member to switch from a open to limited switch?

 

If you are discussing a Ford 9" rear axle whereby you swap out the complete third member, yes. One for the other. I do not know if Ford made such a critter, though. Others more knowledgeable can speak to this.

If you replace the inner workings, gears and all, then you need to have the third member properly setup.

Ford did not offer a 9" with a locking setup in these trucks, at least '65/'66. The locking rear axle would have been the Dana 44 (for F100) or 60 (for F250).
These require nearly complete disassembly to convert from non-locking to locking.

 

if you are just talking about changing the "pumkin" on a 9", it's a simple bolt in. make sure the axel splines are the same, check the yoke too.