Is there a muscular disease that restricts your rib cage from expanding? Poor posture has caused my rib cage to become restricted by the muscles.

Several, but. Several neuromuscular diseases could do this and posture could restrict rib cage expansion, but should not directly affect muscles.
Yes. The simple answer to your question is yes, there are some neuromuscular conditions that can affect they'd led that aid in respiration. They are all pretty rare, however, and conditions such as reactive airway disease are much more common as a cause of difficulty with breathing. Thank you.