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I have a 1999 XLT 4x4 4.6 with 17" wheels and the optional factory towing package that I purchased new (package indicated on window sticker). Axle ratio is 3.55 open differential. I'm under the impression that the 3.73 LS was required when a 1999 Expedition 4x4 was ordered with the 5.4 engine AND 17" wheels. Also the 3.73 was required in a 5.4 4x4 Expedition ordered with the high altitude package. The lowest gearing a 4.6 4x4 could be had with under any circumstances is 3.55.
Furthermore, a specific axle ratio was not part of the spec for the optional towing package.
You may want to go over to www.xpedition.net and download a copy of the 1999 Expedition ordering guide which discusses standard and optional equipment combinations on 1999 Expeditions.
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