The main qualifying point is that you have a college degree. As long as you meet the minimum course requirements you should be fine. During my career I have worked with fellow park rangers with degrees in nuclear physics, Russian Studies, Business Management, Communications, Hotel Motel Management, Education, Physical Education, Criminal Justice, English, Spanish, and many more majors that are not specifically related. Now if you were applying for a specific job as an interpretive ranger or researcher in say a civil war park, they would definitely be looking for a history degree. So I do not believe that your degree field will hamper you in any way. In would think that your military and previous law enforcement experience will highly benefit you in the selection process far out weighing any concerns about your college major.
At 31 years of age you still have a pretty good window of opportunity to meet your plans. The key for a law enforcement park ranger job is that you have to be hired and get to the Federal Law Enforcement Training Center before you turn 37. If you are a viable candidate with the prior experience, any manager that knows they can get you into training before your birth date will not hesitate to seriously consider you.