relkap,
If the problem is intermittent it still could be a number of quite simple things.
As far as fluid, DC recommends using ATF+4.
More than likely, your fluid is ATF+3 but if you did not have this truck from new, then there is no way to know for sure if the fluid had every been changed to ATF+4 (or ever changed at all).
Essentially, your truck came w/ATF+3 and DC says it is preferable to add (if/when needed) ATF+4. If your tranny had its fluid changed to ATF+4 already, then you could NOT go and add ATF+3 fluid (it would gum things up) but adding ATF+4 to existing ATF+3 is perfectly safe (and recommended) according to DC.
As far as where to take your truck, you are in luck. What I mean is that you are not currently stranded so you have a short bit of time to get a recommendation from people you trust. If no one in your family has ever had a transmission repair experience, then you could get resourceful and ask either friends or some folks at church - someone will have something nice to say about a shop in your town.
As far as going to a DC dealer for service, sure that's a good thing but if the tranny needs a rebuild, then you would be better off to go directly to a specialty shop with a good reputation (more than likely, your local DC dealer does not have the equipment necessary to perform a complex transmission repair or rebuild and they end up sub-letting the repair order to one of your better local tranny repair shops anyway).
In my area, most of the DC dealers sublet repair orders (and warranty work is the same) to a local outfit called "Cottman Transmissions" - Cottman is known nationwide.
As far as AAMCO, I don't have any negative experience with them so I couldn't say go to Cottman instead of AAMCO - it's best in your town to get a local recommendation.
Pep Boys or some other chain-based auto supply house should have ATF+4 on hand. Remember that the tranny fluid level needs to be checked with the vehicle as level as possible, with the engine hot (fully warmed up) and idling in Park. Do not overfill beyond the higher dot on the stick (slight hole).
It could be that your oil lines are clogged or that your tranny's pump is marginal... either way, it does not sound like you'll be out major cash like a rebuild. Again, if you rebuild or not depends more on the overall condition of the transmission and how much longer you need it to last.
And don't worry - it's not worth the waste of life energy to do so (worry) - then again, you didn't ask for a philosophical answer so feel free to ignore my rambling(s)....