Thanks, Marcus!

OK, thanks.

That said... I asked a native informant 
(Osakan in her mid-30's) and she agreed 
with the assesment that the two words 
aren't mere synonyms. 郊外 she equalled to 
田舎 while 近郊 to her was just a question 
of distance, e.g. whatever's neae the city 
but not the city proper, for example. This 
kinda matches daijr's definitions. 郊外: 都
市の周辺にあって、森林・田畑などが比較的多い住宅地
区。　近郊：都市や町に近い場所。町はずれ。郊外。
It's a tricky line to draw though, as 
evidenced by daijr's ref to 郊外 in the 近
郊 entry.
Prog also:
東京とその近郊
Tokyo and 「the surrounding districts 
[((文)) its environs]
東京近郊に
in the suburbs of Tokyo／on the outskirts 
of Tokyo

I'll tmake an edit to the 近郊 entry too 
to try and reflect this distinction too.

@@ -18,3 +18,3 @@
-<pos>&adj-no;</pos>
-<gloss>suburbs</gloss>
-<gloss>outskirts</gloss>
+<gloss>suburb</gloss>
+<gloss>residential area on the outskirt of a city</gloss>
+<gloss>commuter belt</gloss>

郊外と近郊とでは，都市との物理的な距離や位置関係は
同じですが，その言葉の有するニュアンスというか，話
者の視点が異なります。 郊外は話者が都市の中央（都
心部）に居り，そこから外側を見ているというニュアン
スです。 つまり，都心部から見た周縁部との距離・位
置関係であり，都心部からは離れているということに力
点があります。
https://detail.chiebukuro.yahoo.co.jp/qa/q
uestion_detail/q133305


7618

It'd be helpful if you mentioned that in 
the initial edit, Nicolas.

The difference in nuance mentioned in the 
chiebukuro link above isn't even touched 
on in daijs-daijr-nikk entries so I'm not 
really sure we need to try and 
differentiate them either.