During conversation, I've seen both words used singularly as a question:
Ehrlich?
And:
Wirklich?
In this context, are they both essentially the same as the English:
Really?
More specifically, in which contexts might these two words not be interchangeable?
"Ehrlich" is about being honest, while "wirklich" is about being real. The difference in usage is practically the same as in English.
For example, someone can be real, but dishonest:
Er ist nicht ehrlich.
means "he is not honest", while
Er ist nicht wirklich.
would mean, that he's not real.
In the example you gave, "ehrlich?" also would literally translate to "honestly?", but it is used often in cases where the English would simply ask "really?". This not wrong, it also depends on the situation. In a philosophical way, "honestly?"/"ehrlich?" can be asked and answered in all cases, while "really?"/"wirklich?" can not always be answered/known.
In that context both mean almost the same, I don't think that there is a distinction. (At least not regarding the intention of the question)
In general wirklich refers a bit more to being true while ehrlich is more like telling the truth.
You can compare them better taking the substantives Wirklichkeit und Ehrlichkeit
Wirklichkeit: [alles] das, Bereich dessen, was als Gegebenheit, Erscheinung wahrnehmbar, erfahrbar ist DUDEN
Ehrlichkeit: das Ehrlichsein (1a); Aufrichtigkeit, Wahrhaftigkeit DUDEN
Note: wirklich is often used to emphasize that you mean something exactly as you say it:
Das ist wirklich toll.
ehrlich:
honestly, not lying, a person is "ehrlich" if he/she is saying the truth.
wirklich:
The noun "Wirklichkeit" means "reality". Somethis is "wirklich" if it really exists. Those people are "wirklich": Arnold Schwarzenegger, Albert Einstein, Julius Cäsar. Those are not "wirklich": Donald Duck, Sherlock Holmes, Jean Luc Picard.
If you are told an unbelieveable story like: "Russians have been on moon before Apollo 11", then, in German, you could ask both, "wirklich?" or "ehrlich?". If you ask "wirklich?" then you want to know if the fact you just heard is true (subject of the question is the fact itself). If you ask "ehrlich?" then you ask, if the storyteller is telling the truth or not (subject of your question is the storyteller)
