Ronin Speed works is where I found out about the Explorer center section. As I understand it (but have no way to verify) the change year was 2007. The reason I used the 2009 year info for you was the predisposition for a manufacturer to make a running change and then have it occur during the production year leaving one set of vehicles with one part and everything after that date with the new part. The 2009 production year was deep enough into the run there should have been no chance of a transition year type miscue.
I went back into the Ford parts catalog and discovered the 2006 to 2010 IRS centers for the Explorers are supposed to all be the same. I also discovered the 2007 and earlier housings are being shown as discontinued by Ford similar to the experience you had with the FRPP part. The link below goes to the Auto Nation Parts page for 2007 Explorer.
AXLE HOUSING for 2007 Ford Explorer|7L2Z4010A
Beginning with model year 2008 and later the page pops up showing the part under a new number but still available. For each new year Ford appears to change the first four characters of the number. The link below goes to the Auto Nation Parts page for the 2009 Explorer.
AXLE HOUSING for 2009 Ford Explorer|8L2Z-4010-A.
The last model year they currently show availability for is the 2008 model year. I have a friend who works for TASCA Ford in their FRPP parts operation who has saved my bacon on multiple occasions. His name is Steve and his work phone is (401) 709-7845. Ask for him by name. Tell him tell him I sent you if you want and explain the part you are trying to find.
He will check to be sure it is what you are describing and you will get a very competitive price. Steve used to run the FRPP parts operation at White Bear before AutoNation bought them. Now he is at TASCA.
For everyone else reading this, Steve can find special parts and save you money on them. He is worth a call for the big parts like this. He is doing all of us a favor so please don't weigh him down with small parts questions and requests.
Ed