If you are leaving service for medical/involuntary terms, have you already transferred your GI Bill?

Soldiers who have at least six years of eligible active service must first agree to serve an additional four years of service. Soldiers who have at least six years of eligible service and do not commit to the additional four years prior to being approved for separation for not eligible to transfer benefits.

Check out the rare exceptions at:
http://www.armyreenlistment.com/teb-exceptions.html