I know for a fact if you join USAF you can choose your job ahead of time. If your job is not stated in your contract, YOU DO NOT HAVE A GUARANTEED JOB!!! Don't believe the recruiter, believe the contract as written. Uncle Sam will hold up his end and expect the same of you.
Don't go in if you don't want to, join because you want to and have a positive attitude about it and you will enjoy the experience. Join because someone else wants you to and you will have a miserable few years.