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My truck is chipped and above 57ish, it won't down shift but it rolls coal and takes right off. An unchipped truck will do this as well (mine did before chipping at lower mph). Not sayining it's healthy but you can try to drop a gear (carefully, manually) and see if it drops a gear. If it does, that's a big clue.
What does your tach read at say 45 and if you throttle heavy, does the needle bounce or just lazily climb?