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battery light indicates a possible problem with the charging system. Your batteries may be perfect, but there could be a problem with the alternator. I would have it checked before I set out on a 200 mile trip in the winter.
Because of the age of the battery I'd say it is just the battery. However, if it's not and you replace just the battery you may reduce the lifespan of the new battery. I don't have my manual with me but from what I remember testing the alternator is very simple.
Your truck would probably make the 200 mile trip on battery power only(if the alternator is bad) if you didn't use any extra electrical- lights, windshield wipers ect. I made it about 100 miles on battery power and that included four engine starts, running lights and wipers. I wouldn't recommend it but I had no choice as I was stuck in the middle of nowhere Sunday night and had to be work the next morning. Also I ended up getting towed the last 75 miles- free with my road service.
There is another possibility and that would be the alternator overcharging. In which case get it fixed immediately- the battery could blow up.
I don't believe a bad battery would cause the lite to come on. It usually indicates a no-charge problem. I wouldn't drive any further than a nearby shop to have the system checked.