2007 Taurus only came with the Vulcan engine. The axle has a round clip at the end of the splined part of the axle. This clip pops into place once the axle is inserted properly into the transmission differential gears. It takes prying oiut with a pry bar to pop it loose. Eother it was not inserted enough into the transmission or the clip is not there and or the transmission part is damaged. Most likely it was not inserted enough for it to pop into place.