If you take 10mg oxy every 4 hours with 2mg of alprazolam every 4 hours, as well, and have been for the amount of time you've stated, I highly doubt diphenhydramine will be significantly dangerous to add. It will definitely enhance the sedation level, and may make it feel as if you've taken a higher dosage of alpraz, but it's still something to be careful with. As long as you know your limits and are careful when adding new meds, you should be alright.
Also, totally random and possibly a rude question so don't answer if uncomfortable, but what are you on that much alprazolam for? I don't think I've ever seen somebody 17 years old on that high of a dose without an SSRI or adjunct therapy in my days at psych hospitals lol. Everybody's different, though.