What’s the difference between a sworn shield and a sworn sword?

A sworn sword is another term for a soldier (usually a knight, but not necessarily as the phrase “knights and sworn swords” has been used before) in service to a lord (it’s been used in the sense of “bannerman” but can also refer to a household guard or knight). 

Sworn shield specifically refers to a bodyguard. 

Thus, all sworn shields are sworn swords, but not all sworn swords are sworn shields.