This question example is from the book "HSK Standard Course 1 (2014)". According to the Chinese Grammar Wiki:
When time, place, manner, or purpose are specified in a sentence, 了 can be used immediately after the verb to indicate completion.
Because of the time specification 昨天上午, I've thought that it would also be possible to put 了 after the verb, so that the question becomes 昨天上午你去了哪儿？. Would it still be considered correct? Does this sound natural at all?