J.M.
wow. i just looked up your state's labor laws around maternity leave and you are actually pretty well protected. first thing i'd do is talk to your doctor about getting a note for your employer, stating the dates that you will be unable to work due to medical reasons (starting at week 39, for example). see the MT government website: http://erd.dli.mt.gov/humanright/employdiscrimpregnant.asp
i feel for you. i have a desk job where i sit most of the day, and can't imagine being on my feet for my job while 36 weeks pregnant.
ultimately though, you want to maintain a positive relationship with your boss, so look for any way to help ease her stress of covering for you. would it help her if you were to work through june, but be able to sit most of the time? or when you return to work, promise her you'll be back as soon as you can, maybe before the 6 weeks (or whenever), if you're feeling up for it? or offer to return to work at 4 (or 6, or 8 weeks) after delivery, in a part-time manner, to help her keep her shifts covered? or, do you know of someone you could refer to cover for the time that you'll be out ? just looking for ideas on how to ease her stress, so she will ease off on you.
best of luck.