275/70/18 is a large 33 - it's 33.14". Keep in mind that the standard "33" is actually 32.8", the standard "34" is 33.9" and the standard "35" is 34.4". So the 275/70/18 sits in between a "33" and a "34". It really is the perfect tire size for the Trail Boss because it will work on stock wheels with no interference issues, and it's close enough to the stock "32" (which is actually 32.1") that you can get away without having to re-calibrate (obviously you have to understand that your speedo, odo, and MPGs will be wrong) if you have a refresh truck.
I recommend this size to ANYONE replacing their stock tires - even if they don't want to upsize. It's like upsizing without the work of upsizing.