If you have db backed up as a single .BAK file then restoring the database is the only way I can think of; there is no way to dig around that file.

Note that you can restore it to another computer and grab the DDL for the stored proc from there.

It's worth noting that Redgate has a product (I believe it is SQL Backup Pro) that allows for object-level recovery. I have used it in the past for this kind of situation.