What do you mean "vague"? The 3.0 will always have "u" in the 8th digit and the 4.0 will always have "x" in that spot. The flex fuel 3.0 may have a different digit though. The only exception I can think of to this is if someone did a cab swap or different engine swap.

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1996 Ford Ranger STX 4x4 4.0L
1983 Honda Shadow VT750C