It's a single tire that can be used in the rear and in the front with the provided spacer. I believe it's just as tall as the stock rear tire, so the differential should be fine.
http://www.fordgtforum.com/forums/showthread.php/26016-Motorcraft-5W-50-full-synthetic?highlight=Rotella
They're switching to Rotella T6 5w40 as they say that Motorcraft 5w50 breaks down too fast.
All the dealers around here were $10k over for non Rs. I had to wait until my local dealer had sat on theirs for 8 months before they would let it go for MSRP.