swagger - ask your military supervisor, flat out, if he/she will write you a good LOR. It's totally legit to ask that. Hopefully, you can count on an honest response that you can trust (i'm hoping that even if they might not like you none of them would lie to you and go out of their way to screw you).
Also, don't worry about whether or not they can say you are good for law school. Adcoms just want to see that your supervisor had great things to say about your performance in relation to the work you did. Now, if they can't write worth a darn that is a concern. But i'm gonna assume that adcoms know that the LOR is coming from a military supervisor and thus are expecting less in terms of eloquence.