The Expedition is basically an F-150 rear end, and the Excursion is an F-250 Super Duty rear end. What are you trying to do? F-150 and Expedition rear-ends are probably the easiest to find, and therefore the cheapest. They offered open differential or traction-lok, and at least 3-4 different gear ratios.
Expedition's went to independent rear suspension in about 2003, so the newer ones would not have a standard straight axle.