Tascaparts.com has them for 49 each, no mention of front or rear. Rockauto has various brands, including Motorcraft (about 53 front, 56 rear) and Bosch (39 upstream, 41 downstream).
Not sure if the $102 and $106 you gave was for each one, or not.
Damn, Matts sniped me while I was looking prices up
EDIT: Oh seems like they do sell them at Advanced Auto. 43 for upstream Bosch, 47 for downstream. Might be cheaper if rockauto's shipping is too much, or if you need them sooner.
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2003 Expedition XLT 4.6 V8
AFE CAI
Custom Sniper Tune by Matt
Swapped Maggy 18" Muffler
Summit underdrive pulleys