Originally Posted by Jerry Bransford
Before getting too deep here, start off with the simplest of basics and go from there... you need to first remove both battery cables and clean them and the battery posts with a wire brush. IF the battery is good like you claim it is, the usual cause for this problem is nothing more than a bad battery connection.
Don't just look at the battery connectors and pronounce them ok either, you really need to remove and clean them with a wire brush and reconnect them solidly before suspecting anything more complex or more expensive.
My TJ and esp the Grand I had were real picky about this. Clean connections and make sure they are tight.
01 TJ, 34 Super Swaper 34x10.5 bias, Winch, locked, belly up, some armor, 4.0, 5spd