There is no diference in these two sentences.But we always say"我们吃完饭了要去哪"or“我们吃完饭后要去哪”完=finish

后and 以后are the same in these two sentences. But I would like to correct them a little: 我们吃完饭后要去哪里？ 我们吃完饭以后要去哪里？ I understand you used了 to indicate the present perfect tense but in this case we always use完, which is also an indicator of present perfect tense.

最荒唐的同音字兼併，莫過于以「后」代「後」了。涇渭分明二千年的兩個漢字，被一群糊塗蟲給搞糊了，也毒害了十三億人民的思想六十年。「后來证实李皇后的后面站的是王皇后的后裔」是為一笑。

in this sentence they are the same.

在这两个句子里，“后”和“以后”意思是完全一样的，they are a same mean, no difference in these two sentences