Can Rangers in Strahd's Possession and Stone Prophet wear chainmail or elven chainmail without incurring a penalty to their dual wielding ability? Second edition rules seem to indicate that they can wear only light armor which would limit them to just leather armor as far as I can tell. Thanks for your time!

Both games used the ADD-version of the Rules, a Ranger can wear all Leather armor without penalty and some rare lightwight medium armor (elven chain, mithril) but i don't remember if any of those rare armor types was in the game.
If you use other armor your Ranger will loose all Classboni for Dualwielding.

Edit
Your Question made me curios and i took a look, Elven Chainmail is in the game and Stoneprophet has even some nice special ligth armor Sets (Egyptian armor). And take into mind that the old ADD-Rules limit Stat-Boni and progression based on Class/Race.

Edit II
Stone Prophet has not lists for levellimits by race, so they might have removed it but both races keep their language,
the higher prime stat and the ability to multiclass.

How would you figure out whether that rule has been implemented in the game or not? Did you run test series?

The infos is from one of the manuals and it's corresponding to the DD Ruleset used by both games.
I played Strahd's a long time ago but i don't even remember my partysetup...if i tested it...i fogott it :)

I'm playing with a ranger right now and it's annoying they don't publish the THAC0 or anything like in baldurs gate. I'm suspicious they never implemented this rule...