I've heard (maybe seen also?) that the Ramban (Rav Moshe ben Nachman, Nachmanides) said that Rabbincal Commands are only obligatory while in the Land of Israel. Is this true? (If so) Where is it recorded? Has it ever been treated by other rishonim/poskim?
I think you are thinking of his commentary to Sefer VaYikra 18:25, where he famously argues, based on the Sifri, that (many) positive commandments are not applicable outside Eretz Yisrael, but are practiced anyway so that they are not forgotten.