The last two letters are just revision numbers and don't matter. Just look for the number portion. BTW, the difference between the to numbers is that one is supposed to be a "premium" model that shows individual tire pressure, but no one on this forum has been able to make that work with an aftermarket install. So for all purposes, just get the one you can find.
As far as finding one, what I did was find scrapyard sellers on eBay that seemed to have 05-06 LTD Liberty models that they were parting out. I then contacted them individually to ask if they had the part and to confirm the specific part number. They almost never come up as auctions. There are also the online services that connect you with scrapyards where you can put a request in, but I haven't tried that.
http://www.lkqonline.com/If you read through the thread I posted above, it is by far the most comprehensive on the topic. There are lots of others around if you google where bits and pieces of information can be found about installation.
Good luck.