If you get the codes and can't find your book just post them here and I'll look them up - I've got the actual list from Dodge. Put the year/make/model/and engine size in the same post with the codes to make the reply easier.
At the mileage you've stated, if that alternator hasn't ever been changed I'd say that's another sign. I'd still do a longer idle test though to be sure so you don't waste the dollars on another repair that won't actually fix it, but I'm getting pretty convinced. If that voltage drops over time I'd say that's the sign. If it throws a code, even better.
Good luck, and keep us posted.