The easiest way to tell is to wet down some pavement and do a drag strip type start, if only one wheel spins then the unit needs work. If both 12 bolts are truck and the axle spline count is the same the carriers should interchange, not a direct bolt in as the pinion depth, tooth pattern, backlash and bearing preload must be set per each individual rear housing. If the 5 lug is a car the unit won't work without major mods.
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