The rear 4x4 blocks are 1.5". I was able to get my Nittos for ~$900 from Sears installed so you should be able to get the 275s for that price. I was going to go with the AT3s but their availability was sparse and the Nittos were my 2nd choice. Check out tirecrawler.com for pricing.
I saw where you said you put in the OEM rear block and was wondering how tall the OEM one is. The guy doing mine said he was put in a 1.5" which is fine I guess but I don't my truck to look like ****. He's putting the 2.5" up front and also, you put 275/70's so I guess I can go with those but I found a great deal on some Cooper Discover ATP's 265/70 out the door for 870.00. Thanks in advance.