Are women allowed to give a d’ver torah on shabbos in shul after morning davening, just as a rabbi would do? If so (or not) where can I find this in halacha?

Shalom, As with many questions of this nature, the halacha gives great value to local custom and practice. There are communities where women take a greater public role and those where their role is less public. This is one of the most important jobs of the Rabbis in each location - to determine what is right and proper for his own community. So, on a very practical level I direct you to your local Rabbi who will be able to explain to you his decision about your shule. For further sources you can see Rav Moshe Feinstein zt"l (Igrot Moshe, O.H. 5, 12 (3)) Blessings